(maint) Require forwardable in puppet/settings.rb

Commit a8e9aa6 extended Puppet::Settings with Forwardable, but did not
require it.  This passed specs, but failed in acceptance when run with
system rubies (locally with rvm/rbenv, we're not seeing a problem).
Requiring forwardable, as we do in other files seems to fix the issue.
Merge pull request #2989 from jpartlow/maint/master/require-forwardable-in-settings

(maint) Require forwardable in puppet/settings.rb