heroku migrate時のNoMethodError: undefined method `tr' for nil:NilClass

###前提・実現したいこと
現在herokuデプロイまでは完了したのですが、その後のheroku run rake db:migrateで詰まってしまいました。
開発環境ではとくにエラーが出ることもなく動作しているので、なにかしらの設定が足りていないのではと考えていますが、どうにもわかりません。
追加で情報などが必要であればメッセージをお願いいたします。

よろしくお願いいたします。

###発生している問題・エラーメッセージ
rake aborted!
NoMethodError: undefined method `tr' for nil:NilClass

###該当のソースコード
rake aborted!
NoMethodError: undefined method tr' for nil:NilClass Did you mean? try /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:37:in initialize'
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:270:in new' /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:270:in resolve_url_connection'
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:258:in resolve_hash_connection' /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:215:in resolve_connection'
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:139:in resolve' /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:151:in block in resolve_all'
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:150:in each' /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:150:in resolve_all'
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/connection_handling.rb:69:in resolve' /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/core.rb:46:in configurations='
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/core.rb:48:in block in <module:Core>' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/concern.rb:120:in class_eval'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/concern.rb:120:in append_features' /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/base.rb:284:in include'
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/base.rb:284:in <class:Base>' /app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/base.rb:269:in module:ActiveRecord'
/app/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/base.rb:26:in <top (required)>' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in require'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in block in require' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:240:in load_dependency'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in require' /app/app/models/admin.rb:1:in <top (required)>'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in require' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in block in require'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:240:in load_dependency' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in require'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:360:in require_or_load' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:494:in load_missing_constant'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:184:in const_missing' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:261:in const_get'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:261:in block in constantize' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:259:in each'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:259:in inject' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:259:in constantize'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:566:in get' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:597:in constantize'
/app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise.rb:313:in get' /app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/mapping.rb:81:in to'
/app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/mapping.rb:76:in modules' /app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/mapping.rb:93:in routes'
/app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/mapping.rb:160:in default_used_route' /app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/mapping.rb:70:in initialize'
/app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise.rb:343:in new' /app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise.rb:343:in add_mapping'
/app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/rails/routes.rb:241:in block in devise_for' /app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/rails/routes.rb:240:in each'
/app/vendor/bundle/ruby/2.3.0/gems/devise-4.3.0/lib/devise/rails/routes.rb:240:in devise_for' /app/config/routes.rb:2:in block in <top (required)>'
/app/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.6/lib/action_dispatch/routing/route_set.rb:432:in instance_exec' /app/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.6/lib/action_dispatch/routing/route_set.rb:432:in eval_block'
/app/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.6/lib/action_dispatch/routing/route_set.rb:410:in draw' /app/config/routes.rb:1:in <top (required)>'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:268:in load' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:268:in block in load'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:240:in load_dependency' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:268:in load'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/routes_reloader.rb:40:in block in load_paths' /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/routes_reloader.rb:40:in each'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/routes_reloader.rb:40:in load_paths' /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/routes_reloader.rb:16:in reload!'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/routes_reloader.rb:26:in block in updater' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/file_update_checker.rb:75:in execute'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/routes_reloader.rb:27:in updater' /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/routes_reloader.rb:7:in execute_if_updated'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/finisher.rb:69:in block in <module:Finisher>' /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:30:in instance_exec'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:30:in run' /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:55:in block in run_initializers'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:54:in run_initializers' /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application.rb:352:in initialize!'
/app/config/environment.rb:5:in <top (required)>' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in require'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in block in require' /app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:240:in load_dependency'
/app/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in require' /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application.rb:328:in require_environment!'
/app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application.rb:457:in `block in run_tasks_blocks'

/app/vendor/bundle/ruby/2.3.0/gems/bundler-1.13.7/exe/bundle:26:in `<top (required)>'


###試したこと
heroku restart
アソシエーションの見直し


###補足情報(言語/FW/ツール等のバージョンなど)
より詳細な情報
DBは mysql2のバージョン0.3.18に設定してあります。
rails', '4.2.6'
ruby 2.1.3
```
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
end

gem 'carrierwave'
gem 'devise'
gem 'gmaps4rails'
gem 'jquery-turbolinks'
gem 'rmagick'
gem 'pry-rails'
gem 'compass-rails','~> 2.0'
gem 'sprockets', '2.11.0'

group :production do
  gem 'rails_12factor'
end
```