I emailed MongoLab support and they said that increased restrictions on the system namespace in Mongo 2.6 are preventing inserting into the system.index collection. Instead, we should be using ensureIndex (see here: http://docs.mongodb.org/manual/reference/system-collections/).
I'm getting an error when I try to push to Heroku (with MongoLab).
I emailed MongoLab support and they said that increased restrictions on the system namespace in Mongo 2.6 are preventing inserting into the system.index collection. Instead, we should be using ensureIndex (see here: http://docs.mongodb.org/manual/reference/system-collections/).