Add testing against HHVM at Travis-CI
minor #9761 Add testing against HHVM at Travis-CI (stloyd)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #9761).

Discussion
----------

Add testing against HHVM at Travis-CI

Commits
-------

a4d1095 Add testing against HHVM at Travis-CI