travis - PHP 5.6 added, linter added

I used also for DoctrineBehaviors's port to Nette: https://github.com/Zenify/DoctrineBehaviors/blob/master/.travis.yml

See: https://github.com/KnpLabs/DoctrineBehaviors/commit/7ee1e2877a039b4d080da67d723a65532c44837b#commitcomment-8186595

See: https://github.com/JakubOnderka/PHP-Parallel-Lint