Add capability to use URI to allow for Atlas compatibility

[ebarojas]

This PR allows the use of URI instead of user names and passwords, which allows to interact with MongoDB Atlas sharded M0 free tier databases. Since the removal of the mLab add-on, this is the suggested Heroku workflow.

PS: Unsure of the etiquette with your repo, I realize this would require changing your local config.yml however I do hope this is useful. Thanks so much for this @sheharyar, we use this a lot.

Referenced in issue #26

[malinda1986]

Hello, why is this not merged, works perfectly 🎉