Make tests run, and remember which bundle made it possible

[pablobm]

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, Bundler must have used ActiveSupport version 3.x-ish
4. ActiveSupport started depending on Minitest from 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. ⚡ 💥 🔥

So I un-gitignore'd Gemfile.lock, allowed Mocha and Shoulda to upgrade, and converted the tests to Minitest.