Install the mainline version of Nginx

In the world of Nginx, mainline can be considered the most feature rich and the most stable. The "stable" release receives only bug updates. A good explanation of this was posted by the nginx project after the initial 1.7.x release from 1.6.x - http://nginx.com/blog/nginx-1-6-1-7-released/

I've used nginx 1.7.x in production since that day and have not had any troubles. I think we should be good to move to mainline as our default provisioning.

The way provisioning works in VVV, this will only be noticed during the initial `vagrant up` and then after running `apt-get update`, `apt-get install nginx` inside the VM. In the future, It would be nice to mark several packages as always updates or provide a script to run to update these when a solid Internet connection is available.