ruby on rails tutorial を勉強中なのですがherokuにpush出来ません

ruby on rails tutorial を勉強中なのですがherokuにpush出来ません。エラーはこちらになります

Counting objects: 26, done.
Compressing objects: 100% (26/26), done.
Writing objects: 100% (26/26), 3.96 KiB | 579.00 KiB/s, done.
Total 26 (delta 16), 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-2.4.4
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 (1.16.5). We suggest you upgrade to the latest version of Bundler by running gem install bundler.
remote: Fetching gem metadata from https://rubygems.org/.........
remote: Fetching version metadata from https://rubygems.org/..
remote: Fetching dependency metadata from https://rubygems.org/.
remote: Could not find concurrent-ruby-1.1.0 in any of the sources
remote: Bundler Output: Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.16.5). We suggest you upgrade to the latest version of Bundler by running gem install bundler.
remote: Fetching gem metadata from https://rubygems.org/.........
remote: Fetching version metadata from https://rubygems.org/..
remote: Fetching dependency metadata from https://rubygems.org/.
remote: Could not find concurrent-ruby-1.1.0 in any of the sources
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 kaigo-log.
remote: