Add minitest to Gemfile

In order to use the system Fedora/EPEL ruby we need to install minitest in production. There is an old issue which this PR fixes it: #3783.

Long story short is that the packaged ruby in Fedora/RHEL/CentOS doesn't ship with minitest, it is packaged as a separate library.

This was the case with GitLab and Discourse where both now include minitest in Gemfile.

https://github.com/gitlabhq/gitlabhq/issues/3826
https://github.com/gitlabhq/gitlabhq/pull/3852
https://github.com/discourse/discourse/pull/238

We use system ruby on CentOS7 and the modified Gemfile at https://librenet.gr without any issues so far.