(maint) No longer necessary to gem install rgen

Puppet used to express gem dependencies on rgen in its Gemfile, packaging,
etc, but in PUP-2914 the rgen code was vendored into puppet, and the
dependencies removed.

However, the future parser setup step still tried to install the rgen
gem, but only when testing against git checkouts (effectively Solaris
and Windows).

Since rgen is vendored in puppet, it is not necessary to install the gem
during future parser testing.
Merge pull request #3365 from joshcooper/maint/stable/remove-rgen-install

(maint) No longer necessary to gem install rgen