Add MONGODB_URI also

[dclipca]

As I see, colyseus-social checks only for the MONGO_URI environmental variable. It would be better to add also MONGODB_URI so that it will work with the MongoDB addon on Heroku out-of-the-box.

[seiyria]

I think it would be better to move to using MONGODB_URI entirely, instead of supporting two that do the same thing.