How do I fix a Bundler Conflict when pushing to Heroku?

I'm attempting to deploy a Sinatra app to Heroku. Everything works locally. However I get an issue with bundler when I try to push it successfully to Heroku.

I've tried to delete the Gemfile lock and then the following

bundle update
git add .
git push "Gemfile.lock"
git push heroku master

gem install bundler:2.0.1
bundle install
git add .
git push "Gemfile.lock"
git push heroku master

bundle update --bundler
git add .
git push "Gemfile.lock"
git push heroku master

Here is my gemfile

source "https://rubygems.org"

# gem "rails"
gem 'sinatra'
gem 'activerecord', '~> 5.2', '>= 5.2.2.1'
gem 'rake'
gem 'sinatra-activerecord'
gem 'require_all'
gem 'bcrypt'
gem 'rack-flash3'
gem 'rb-readline'

group :development, :test do
 gem 'sqlite3'
 gem 'shotgun'
 gem 'pry'
end

group :production do
 gem 'pg'
end

Here are the error messages

remote: -----> Ruby app detected
remote: -----> Compiling Ruby/Rack
remote: -----> Using Ruby version: ruby-2.5.5
remote: -----> Installing dependencies using bundler 2.0.2
remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
remote:        Activating bundler (2.0.1) failed:
remote:        Could not find 'bundler' (2.0.1) required by your /tmp/build_77505ef74596fc4dddd66c8b792d759b/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:2.0.1`
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 '2.0.1'`
remote:        Bundler Output: Activating bundler (2.0.1) failed:
remote:        Could not find 'bundler' (2.0.1) required by your /tmp/build_77505ef74596fc4dddd66c8b792d759b/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:2.0.1`
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 '2.0.1'`
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 infinite-waters-69714.
remote: 
To https://git.heroku.com/infinite-waters-69714.git
 ! [remote rejected] master -> master (pre-receive hook declined)

Additionally, I've tried to remove sqlite3 altogether and repeat the above processes.

Any ideas on how to fix this?

Solution

I've ran into this issue some time ago: I contacted Heroku and they told me they have a locked version of Bundler, so your only option is to use the Bundler version they use:

gem uninstall bundler
gem install bundler -v 2.0.2
bundle update