(GH-139) Throw away STDERR from dpkg-query in Fact

Newer versions of dpkg-query, as of Ubuntu 12.10, will make noise on STDERR
if the queried package isn't currently installed. Facter's `exec()` outputs
this without giving us a chance to catch it.

Pipe STDERR to `/dev/null` so that it's not seen by the end-user. STDOUT
will still be `nil` if the package isn't installed. It doesn't seem
reasonable to spec test for this without reaching deep into Facter, so I'm
not going to.