Add Rails 4 support.

Mix shoulda-context in MiniTest::Spec if present and part of ActiveSupport::TestCase's ancestors.

This is a simple suggestion for adding support to upcoming Rails 4, which now has its base test class inheriting from MiniTest::Spec instead of Test::Unit::TestCase.

I'm not expecting this pull-request to merged right away, as it doesn't include any tests or Appraisal support for Rails 4, but I'd love to hear your thoughts on this approach.