Define Symbol#intern if missing.

Ruby 1.9 defined Symbol#intern as return self, which makes perfect sense.
There is no equivalent on 1.8, which means that we can hit some issues now
that we return symbols for 1.9 compatibility in places.

Since the definition is obvious and correct, we may as well just implement the
missing method.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>