Drop Rubinius from Travis CI tests

Because it seems like way too much work.

'I don't care about Rubinius. Evan knows how I feel' - Ben
Simplifying Gemfile minitest dependency

Since the test Gemfile group only contains the single minitest gem, we
can switch over to passing a group parameter to the minitest gem
dependency instead of explicitly creating a group.
Adding myself to contributers

My only goal in life has been to become a contributer to BenString, and
with this commit my dreams will have finally come true.
Still bad at typing

Guess I love the enter key too much