Consider dropping HHVM support

[dominics]

Builds for HHVM are currently failing.

Presumably should be a major version to drop support, so maybe consider just no longer testing on HHVM for now and officially dropping support later.

Nobody is using it (happy to be surprised if that's not the case), and it's still not compatible with 7.0 features (let alone 7.1, and 7.2 is now in alpha)