Updates version checking to deal with HHVM nightly builds

The HHVM nightly builds have a different versioning scheme, nightly-yyyy-mm-dd

This temporary patch just scans to see if it is a nightly build and if so, uses a fixed string, 2.4.0-dev which I pulled from the current master branch of hhvm.

I'm trying to run a set of unit tests up on travis for the joomla framework and ran into some issues, so I tried upgrading it to the nightly build and now composer blows up when checking the php version.  

Feedback appreciated on a better set of logic to convert nightly builds to some dev version[maybe parse the date and pull the version string directly from the hhvm repo by date lookup?]