heroku mysqltopostgres installation error - mysql

Question

I am trying to deploy my php app that uses mysql db to heroku.
I followed instructions here - https://devcenter.heroku.com/articles/heroku-mysql
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
''
Has anyone ever come across these errors?
And I am new to Heroku, and I was building my app on my local apache and now I want to deploy it to heroku.