Drop support for HHVM and add PHP 7.2 in Travis CI

[tortuetorche]

HHVM isn't support anymore by Laravel since version 5.4

[tortuetorche]

The Travis CI builds failed because of a

[Composer\Downloader\TransportException]                                                                               
The "http://packagist.org/p/provider-2017-07%24423ecf739f4447f3d9e415b097c03d615a5af9e228dd63f574b7ce3298c81294.json" file
could not be downloaded (HTTP/1.1 404 Not Found)

I'll send another dummy commit, to retry the build...

[PatrickRose]

Thanks!