Fix Rails 4 support for rc1:

ActiveSupport::TestCase now inherits from MiniTest::Unit::TestCase instead of MiniTest::Spec. Please see: https://github.com/rails/rails/commit/eb4930e3c724cf71d6ce5bb2aec4af82b2025b03
Merge pull request #23 from melindaweathers/master

Support for Rails 4 RC1