Refer developers to the vagrant vm and remove out of date documentation.
Created by: dosire
The best way to develop for Gitlab is to use the GItlab Vagrant virtual machine. This PR updates all the documentation to point to the vm. This will make it easier for new people to contribute.
Existing documentation that is out of date (the list of packages to install) or that doesn't work on the VM (using foreman to start gitlab) is removed.