Switch for forge modules for fixtures

Now some months after my submission `puppetlabs_spec_helper` was
accepted and eventually released, I am now able to actually use the
feature. And due to a (presumably) spurious failure connecting to
Github, I am motivated to actually use it.

I am also dropping the requirement for stdlib 2.6.0, since (currently)
all of the released versions are compatible w/Puppet 2.7.