(PUP-1879) Clear gem path cache when loading library features

Installing gems during a Puppet run should be picked up by features using the
'libs' option.  However gems are installed into separate directories, so
rubygems maintains an in-memory directory cache, which needs invalidating
when attempting to load a library after installation.
Merge pull request #2415 from domcleal/tickets/master/pup-1879-rubygem-cache

(PUP-1879) Clear gem path cache when loading library features