add ruby '2.6.6' in gemfile if heroku errors on deploy

remote: -----> Detecting rake tasks remote: remote: ! remote: ! Could not detect rake tasks remote: ! ensure you can run$ bundle exec rake -Pagainst your app remote: ! and using the production group of your Gemfile. remote: ! /tmp/build_3df62780/config/boot.rb:4:inrequire': cannot load such file -- bootsnap/setup (LoadError)
remote: ! from /tmp/build_3df62780/config/boot.rb:4:in <top (required)>' remote: ! from /tmp/build_3df62780/bin/rake:7:inrequire_relative'
remote: ! from /tmp/build_3df62780/bin/rake:7:in <main>' remote: !