Add Linux support

This adds a basic Linux provider, as requested in [MODULES-1560](https://tickets.puppetlabs.com/browse/MODULES-1560).  It does not presently update the spec tests to handle that.  Suggestions on which tests to look at fixing are welcome.
Truth be told, it's probably not.  It was a shot in the dark at dealing with the stack level too deep error caused during the Travis builds.  It didn't work.