[Docker image] - MONGOLAB_URI variable deprecated

[pascoal-g]

Hi, first of all congratulations for the project, I'm a fan.

I run pisignage-server on kubernetes, and to configure the connection between Server and mongoDB, I use the variable MONGOLAB_URI. It was discontinued in the lastest image of docker on config/env/production.js d84f6b12f10cbb3ed27f88af24de9a55df068a8a

I think this is a good feature to keep.