Update all the test suite

Move from minitest to rspec
Add capybara-webkit for headless tests
Use spork to fasten the tests of the development environment
Remove the gems from the Gemfile. Now using the gemspec method
Update all the tests to use the rspec expect syntax
Now adding the VCR cassettes to the test suite
Still got an issue while running 3 of the tests:
- Card.delete
- Contribution.refund
- StrongAuthentication.get