Use bundle exec when running tests for all gemfiles

Refactor script a bit, improve output.
Create our own TestCase class to add methods

Rather than extending the base Test::Unit::TestCase, simply create our
own test case and add methods to it, and make sure we inherit from this
new test case everywhere.

Also make sure to call `super` on the teardown method.
Test against latest Ruby 2.1 available in travis
Update travis badge and link to the project on travis [ci skip]
Fix Gemfile dependencies for Rails 4.1 and use Minitest if available

Fallback to test/unit in case minitest is not available, which should be
necessary for Ruby 1.8.7 only.