Ruby on rails の　 git push heroku masterした時のエラーの対処法

Ruby on rails でgit push heroku masterをした際に、このようなエラーで出ます。

Counting objects: 118, done.
Compressing objects: 100% (109/109), done.
Writing objects: 100% (118/118), 13.62 KiB | 516.00 KiB/s, done.
Total 118 (delta 77), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Ruby app detected
remote: -----> Compiling Ruby/Rails
remote: 
remote: ###### WARNING:
remote: 
remote:        This buildpack was created as a stop-gap measure to allow running applications with Bundler 2 on Heroku.
remote:        Heroku now supports Bundler 2 directly: https://devcenter.heroku.com/changelog-items/1563
remote:        
remote:        Please discontinue use of this buildpack and instead directly use the `heroku/ruby` buildpack.
remote:        
remote:        To remove this buildpack use the `heroku buildpacks` command to list your existing buildpacks.
remote:        
remote:        If you only have one buildpack listed you can run:
remote:        
remote:        ```
remote:        heroku buildpacks:set heroku/ruby
remote:        ```
remote:        
remote:        If you have multiple buildpacks, you'll need to add the buildpack to the correct location using
remote:        `heroku buildpacks:add heroku/ruby -i <correct index>` and then remove this buildpack via:
remote:        
remote:        ```
remote:        heroku buildpacks:remove https://github.com/bundler/heroku-buildpack-bundler2
remote:        ```
remote: 
remote:        Your app was upgraded to bundler 2.0.1.
remote:        Previously you had a successful deploy with bundler 1.17.3.
remote:        
remote:        If you see problems related to the bundler version please refer to:
remote:        https://devcenter.heroku.com/articles/bundler-version
remote:        
remote: -----> Using Ruby version: ruby-2.5.3
remote: -----> Installing dependencies using bundler 2.0.1
remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
remote:        Activating bundler (2.1.4) failed:
remote:        Could not find 'bundler' (2.1.4) required by your /tmp/build_d444bfd4d727a3ae0cd15c4135d32319/Gemfile.lock.
remote:        To update to the latest version installed on your system, run `bundle update --bundler`.
remote:        To install the missing version, run `gem install bundler:2.1.4`
remote:        Checked in 'GEM_PATH=vendor/bundle/ruby/2.5.0', execute `gem env` for more information
remote:        
remote:        To install the version of bundler this project requires, run `gem install bundler -v '2.1.4'`
remote:        Bundler Output: Activating bundler (2.1.4) failed:
remote:        Could not find 'bundler' (2.1.4) required by your /tmp/build_d444bfd4d727a3ae0cd15c4135d32319/Gemfile.lock.
remote:        To update to the latest version installed on your system, run `bundle update --bundler`.
remote:        To install the missing version, run `gem install bundler:2.1.4`
remote:        Checked in 'GEM_PATH=vendor/bundle/ruby/2.5.0', execute `gem env` for more information
remote:        
remote:        To install the version of bundler this project requires, run `gem install bundler -v '2.1.4'`
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 blablabla.
remote: 
To https://git.heroku.com/blablabla.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/blablabla.git'