Make tests run, and remember which bundle made it possible

Wanted to work on a potential pull request, but tests were not running at all :-( This pull request brings tests back to life.

I can't be 100% sure of what the problem was, but I think it's the following:

  1. `Gemfile.lock` was in `.gitignore`
  2. As a result, a recently cloned copy of Canable would use a latest ActiveSupport when bundled
  3. When last tested developers, the bundled ActiveSupport must have been version 3.x-ish
  4. ActiveSupport started depending on Minitest starting with version 4.0.0
  5. The tests are written using Test::Unit
  6. Shoulda and Mocha both were pinned to old versions that did not deal well with both Minitest and Test::Unit being present
  7. :zap: :boom: :fire:

So I un-gitignore'd Gemfile.lock, allowed Mocha and Shoulda to upgrade, and converted the tests to Minitest.