Add minitest/unit via shims for test/unit.

* Specify minitest ~>4.2.0 for rails < 4 to avoid breaking changes.
* Peg mocha/minitest to old versions for cross-compatibility with ActiveSupport.
* Conditionally require mocha integration for different versions of ruby: http://jamesmead.org/blog/2009-02-02-test-unit-and-minitest-with-different-ruby-versions