bash: rails: command not found　と出てしまいます・・・

 前提・実現したいこと

rails5で作ったものをHerokuにデプロイしたいです！

 発生している問題・エラーメッセージ

heroku run rails db:migrateをすると、

Running rails db:migrate on ⬢ bookchange-090... up, run.8258 (Free)
bash: rails: command not found

というエラーが出てしまいます。

 Gemfie , database.yml

Gemfie

group :production do
  gem 'pg', '0.21.0'
end

database.yml

production:
  adapter: postgresql
  encoding: unicode
  pool: 5
  database: message-board_production
  username: message-board
  password: <%= ENV['MESSAGE-BOARD_DATABASE_PASSWORD'] %>

のようになっています。

 試したこと

git push heroku master をすると、

Counting objects: 228, done.
Compressing objects: 100% (202/202), done.
Writing objects: 100% (228/228), 678.53 KiB | 11.90 MiB/s, done.
Total 228 (delta 70), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Ruby app detected
remote: -----> Compiling Ruby/Rails
remote: -----> Using Ruby version: ruby-2.4.5
remote: -----> Installing dependencies using bundler 1.15.2
remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
remote:        Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.16.3). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
remote:        You are trying to install in deployment mode after changing
remote:        your Gemfile. Run `bundle install` elsewhere and add the
remote:        updated Gemfile.lock to version control.
remote:        
remote:        The dependencies in your gemfile changed
remote:        
remote:        You have added to the Gemfile:
remote:        * pg (= 0.21.0)
remote:        
remote:        You have deleted from the Gemfile:
remote:        * pg (= 0.20.0)
remote:        Bundler Output: Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.16.3). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
remote:        You are trying to install in deployment mode after changing
remote:        your Gemfile. Run `bundle install` elsewhere and add the
remote:        updated Gemfile.lock to version control.
remote:        
remote:        The dependencies in your gemfile changed
remote:        
remote:        You have added to the Gemfile:
remote:        * pg (= 0.21.0)
remote:        
remote:        You have deleted from the Gemfile:
remote:        * pg (= 0.20.0)
remote: 
remote:  !
remote:  !     Failed to install gems via Bundler.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !       Push rejected to bookchange-090.
remote: 
To https://git.heroku.com/bookchange-090.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/bookchange-090.git'

と出てしまいます。

どなたかお詳しい方、ぜひご教授ください！