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.
Curious why this is required. If we are executing from Ruby already, it should be using the correct one you want, so just look it up. 

There is no guarantee which will return the proper ruby that you might be using.