Change nginx_version scheme

This changeset changes the version scheme for the Nginx installation from package version numbers to PPA type.

Whenever a new package is released for Nginx in the PPA used by this role, it doesn't get added to the repository, instead it replaces the existing package. This becomes problematic if you are attempting to pin
package versions.

The best alternative I could come up with was to pin just the repository type (development or stable). This way, if package version number `1.6.2-2+trusty0` gets replaced with `1.6.2-4+trusty0`, the role doesn't break.