Enabled testing on HHVM on Travis

The tested Symfony versions have also been updated.
Added the external code coverage for Scrutinizer
Added the scrutinizer badges

[ci skip]
Fixed url of the Travis badge

[ci skip]
Fixed the composer requirement to allow 1.6
Implemented the support of screenshots
Merge pull request #33 from stof/screenshot

Implemented the support of screenshots
Removed the otherMouseEvents tests

This is not needed anymore as the tests have been split into different
methods in the Mink driver testsuite