Updated the phpdoc to use inheritdoc
Cleaned the phpunit config file
Simplified the Travis configuration

Given that MinkBrowserKitDriver is already tested against the different
symfony versions and there is no code calling Symfony directly in
MinkGoutteDriver anymore, testing against the different Symfony versions
is wasting Travis resources.
Merge pull request #38 from stof/phpdoc

Phpdoc changes and Travis cleanup