Run tests on min version

Since HHVM 3.9 is supported, the typechecker should pass on it as
well as the current version of HHVM. This duplicates the hack test
case to have a version that runs on HHVM 3.9.
2 jobs from master in 1 minute and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #581
hhvm
hack

00:00:29

passed #582
hhvm-3.9
hack-min

00:00:37