Gemfile: Update Rake and Mocha. Remove minitest-colorize

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
```

* Remove minitest-colorize in favor of minitest-reporters.
* Use twiddle-waka for Minitest's version to ensure safe version updates

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