Travis defaults 'hhvm' to 3.6.6 for back-compat, however newer
versions are available. The latest supported version on Travis is 3.12,
per <https://docs.travis-ci.com/user/languages/php#HHVM-versions>.
This also matches the version currently used in wmf production.
Bug: T75175
Change-Id: I1c562f61acf83aa7f558b2e11a6af81d13523dea
- env: dbtype=postgres
php: 5.5
- env: dbtype=mysql
- php: hhvm
+ php: hhvm-3.12
- env: dbtype=mysql
php: 7