(CFACT-44) More fixes for Ruby compatibility.

* Adding a Facter.version method.
* Adding support for booleans in confines.
* Implementing exec and which on Facter::Util::Resolution.

There's one more bigger change to unblock testing of CFACT-44 (relating to gutting the gem in favor of using the Facter compatibility layer), but these are the smaller and easier to review ones.