Fix for "uninitialized constant MiniTest" error

This fixed `/Library/Ruby/Gems/2.0.0/gems/beaker-1.12.0/lib/beaker/test_case.rb:47:in ``<class:TestCase>': uninitialized constant MiniTest (NameError)` error which I was getting on **Mac OS X 10.9.3**, with system **Ruby 2.0.0p451**, **minitest 5.3.4**.

I haven't tested it with any other ruby/minitest combination.