Added hhvm-nightly in the allowed failures on Travis
Added hhvm-nightly in the allowed failures on Travis

This should be reverted once https://github.com/facebook/hhvm/pull/3233 is
fixed.

jakari/prophecy,ftrrtf/prophecy,Taluu/prophecy,dantleech/prophecy,jakzal/prophecy,kamioftea/prophecy,phpspec/prophecy