Can't currently host Slices for free on Heroku #130
Description
Two things on this:
a) The guides are now incorrect, as MongoHQ is no more - MongoLab has taken its place.
b) The Mongo gem is not of a recent enough version to host on MongoLab's free tier (which uses MongoDB 3.0.7) - we'd need to use >= 2.0 to regain that. On paid tiers, you can choose an older version of MongoDB, but I feel there's worth in having a free option!