[BUGFIX] Do not PHP-lint the files in vendor/.
[BUGFIX] Do not PHP-lint the files in vendor/.

The PHP lint in .travis.yml checked _all_ PHP files, including those
in vendor/ (which are not our responsibility).

This change switches the lint to only check the files in the Classes/
and Tests/ directories.

OzzyCzech/emogrifier,MyIntervals/emogrifier,ernsto/emogrifier,Seldaek/emogrifier,iFixit/emogrifier,Synchro/emogrifier,jjriv/emogrifier,sanderkruger/emogrifier,ricardofiorani/emogrifier