Use more lenient version selector for HHVM

Support for 3.2.0 is coming soon, and 3.1.0 may eventually be dropped, so this is better. "Real" version selectors like this one are supported now; no more warning will occur.
Merge pull request #6 from dzuelke/patch-1

Use more lenient version selector for HHVM
Upgrade to Jetpack 3.1
Update composer lock file