Package not tested on PHP 7.0, 7.1, HHVM

[akeeman]

The package is currently not tested with PHP 7.0, 7.1, and HHVM.

• tests on PHP 7.0 don't work because no phpunit is available, and the current test setup doesn't work with phpunit 5.7
• PHP 7.1 has not beed added to .travis.yml file, and if it were, see the point above.
• HHVM isn't tested, because "HHVM is no longer supported on Ubuntu Precise. Please consider using Trusty with dist: trusty."

Maybe someone's got some time to update the test stuff, so it at least works with PHP ^7.0? 😅

[akeeman]

See #157 and #158