[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.
Merge pull request #74 from jjriv/bugfix/php-lint

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