When deploying to Heroku with "Deply to Heroku" button, MongoLab env variable is not correctly set

[karllhughes]

This is probably due to MongoLabDB changing the name of their connection string variable, but when I deployed to Heroku, I got a connection error. Turns out, the variable that was set was not named correctly.

They use MONGODB_URI

This app requires MONGOLAB_URI

The easy switch would be to change it in this app's config.js file, but I didn't want to suggest that as it's a breaking change, so maybe a documentation note?

Either way, happy to make a PR. Thanks!