Add minitest as a dev dependency and fix Minitest warnings

Builds on #3 and fixes the Ruby 2.2.0 build.

Turns out [Ruby 2.2 removes minitest from the ruby build](https://www.ruby-lang.org/en/news/2014/12/25/ruby-2-2-0-released/). It's still included w/ the source, but must be compiled separately from the Ruby build. So, the fix is to add minitest to the gemspec's development dependencies and install it in its gemified form. The version of minitest that I pulled down was issuing a bunch of warnings so I fixed those as well.

[Successful Travis build](https://travis-ci.org/tdg5/directory_monitor/builds/45158750)