Verify, that only specific input/button elements can submit forms
Enabled testing on HHVM and updated the Travis build matrix

Closes Behat/Mink#444
Removed testing on Symfony 2.5 as Silex is not compatible yet
Merge pull request #42 from aik099/any-input-can-submit-form-fix

Ensure, that only specific input/button types are allowed to submit forms