Update minitest

This updates the ore Minitest generator to use the newer `Minitest` name from version 5 rather than the 4.x `MiniTest`. I noticed that the spec for the Gemfile was still checking for `:rubygems` so this has been updated as well. All specs are passing.

Minitest commit notes:
* Uses autorun so minitest/unit and minitest/spec will work.
* Also adds a little pride to the test output.