heroku mysqltopostgres installation error #93

I am trying to deploy my php app that uses mysql db to heroku. I followed instructions here -

The sudo gem install mysqltopostgres-0.3.0.gem command executed without errors:

gem build mysqltopostgres.gemspec

Successfully built RubyGem

Name: mysqltopostgres

Version: 0.3.0

File: mysqltopostgres-0.3.0.gem

but when I try to execute the mysqltopostgres command, the following errors occur:

mysqltopostgres /usr/lib/ruby/vendor_ruby/bundler/runtime.rb:34:in 'block in setup': You have already activated test-unit 2.5.5, but your Gemfile requires test-unit 3.2.1. Prepending 'bundle exec' to your command may solve this. (Gem::LoadError) from /usr/lib/ruby/vendor_ruby/bundler/runtime.rb:19:in 'setup' from /usr/lib/ruby/vendor_ruby/bundler.rb:92:in 'setup' from /usr/lib/ruby/vendor_ruby/bundler/setup.rb:8:in '' from /usr/local/lib/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in 'require' from /usr/local/lib/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in 'require' from /var/lib/gems/2.3.0/gems/mysqltopostgres-0.3.0/bin/mysqltopostgres:5:in '' from /usr/local/bin/mysqltopostgres:22:in 'load' from /usr/local/bin/mysqltopostgres:22:in ''

I then tried to prepend bundle exec:

bundle exec mysqltopostgres /usr/local/lib/site_ruby/2.3.0/rubygems.rb:270:in 'find_spec_for_exe': can't find gem mysqltopostgres (>= 0.a) (Gem::GemNotFoundException) from /usr/local/lib/site_ruby/2.3.0/rubygems.rb:298:in 'activate_bin_path' from /usr/local/bin/mysqltopostgres:22:in ''

Heroku devs told me post an issue here. Thanks in advance