travis: Remove HHVM from allow_failures

HHVM v3.3.0 fixes the unrelated problems we've been seeing.

Deployed on Travis CI as of 14 October 2014
http://blog.travis-ci.com/2014-10-08-october-2014-build-environment-update/
Merge pull request #38 from Krinkle/travis

travis: Remove HHVM from allow_failures