Add minitest/unit via shims for test/unit.

* Specify minitest ~>4.2 for rails < 4 to avoid breaking changes.
* Conditionally require mocha integration for different versions of ruby: http://jamesmead.org/blog/2009-02-02-test-unit-and-minitest-with-different-ruby-versions
* Peg mocha to very old version for its compatibility.