Bump package_version

Summary: Set to 3.5 on the master branch
Closes https://github.com/facebook/hhvm/pull/4178

Reviewed By: @JoelMarcey

Differential Revision: D1669146

Signature: t1:1669146:1415649436:23c7e004b34cc525b29d9897834de48bc43a0eb7