Update gitter channel name
Update gitter channel name

The old channel goes back to our old repository name, and all new Gitter
channels are under out GitHub/Gitter community namespace `readthedocs`.
There are a few channels there, but this is the most general channel.

rtfd/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org