Add spec testing for face component

Prior to this commit there were no unit tests for the Puppet face
piece of this module. Now some spec tests have been adding mainly
around error checking. Due to the use of `puppet module list`, the
behavior of the `modules` and `server` actions are not very
feasible to test via spec testing.

A few minor changes were also made to the gem file, and small change
was made in `check_required_features` to reflect the fact that
this module will no longer support anything earlier than Ruby 1.9.