Revert 2a89c31

The change to the jre config broke builds and upon further review, the
way in which jre versions are hard-coded will prevent projects wishing
to override the default_version provided here (which doesn't align with
the hard-coded version causing some of the breakage issues).

So for now, we're reverting this to get back to a working state.
Add libffi dependency for Debian to ChefDK
Link to omnibus-ruby from README [trivial patch]
test for rhel 5 gcc when building dep-selector-libgecode
improve note on gcc44 and don't shell out in dep-selector-libgecode
go back to shelling out to test for presence of gcc44.