Added script to install MongoDB

#### What's this PR do?
Runs a script to install MongoDB on `$ vagrant provision`.

The `mongodb.sh` script was taken from [Vaprobash](https://github.com/fideloper/Vaprobash/blob/master/scripts/mongodb.sh).

Tested successfully with a local install of the [user-api](https://github.com/DoSomething/user-api) project.

#### Any background context?
The **user-api** will be using Mongo, and it's foreseeable that other future Laravel projects will too. So ... here you go!

cc: @weerd @DFurnes @angaither