Fix Appraisals usage
Upgrade Rails dependency to 4.0.3
Add tests for multiple test framework support
Fix compatibility with Minitest 5 / Rails 4.1

* Rails 4.1 switched from MiniTest 4 to Minitest 5, and
  MiniTest::Unit::TestCase omits a warning under Minitest 5 (the correct
  constant to use now is Minitest::Test).
* Minitest will only run tests that start with 'test_'. This was a
  change from MiniTest 4, which allowed tests to start with 'test'. This
  means that tests defined with shoulda-context no longer get executed
  under MiniTest 5, because tests there start with "test:".