Gemfile: Update Minitest-colorize, Rake, and Mocha

All gems are now up to date. The following changes were copied from the
gem's respective changelog that may pertain to us:

Rake changes:

* The rake task arguments can contain escaped commas (10.1)
* Restored Ruby 1.8.7 compatibility (10.2)

Mocha changes:

* Consider stubs on superclasses if none exist on primary receiver. Largely based
  on changes suggested by @ccutrer in #145. Note: this may break existing tests
  which rely on the old behaviour

Minitest-Colorize Changes:

* Supports Minitest 5.3
* Original gem has not been upgraded since Feb 19, 2013 but a PR was
  made to allow Minitest 5.3. We are now tracking that branch

On my local machine, I was able to shave 6 seconds off the test suite.