Make specs work with Ruby 2.0 and 2.1

This PR makes it possible to develop against Ruby 2.1.1 locally.  The specs are passing with the below updates run against 2.1.1.  I'd be curious to see if ci passes as well.

  * Update Rakefile to not error on unknown Ruby
  * Add explicit minitest development dependency
  * Require minitest in place of test/unit
  * Make test output colorful
  * Change assert_not_nil to refute_nil