hhvm: create module + list all dev dependencies

Ori requested Jenkins to build extensions with HHVM. To do so we need a
bunch of packages dependencies he listed up on bug 63120.

This patch introduces a hhvm module and a single class
hhvm::packages::dev which instruct apt-get to install the build
dependencies for the hhvm package.  That is nicer than attempting to
manually maintain a list of package dependencies.

Include it on labs Jenkins slaves running Trusty.

We might be missing some packages, in such a case they probably need to
be added to the hhvm package build-dependency field.

Bug: 63120
Change-Id: I7c271d65a0856c0d21057618e6eb59ad1bdf421e