fixes #83 by wrapping contents of pkgrepo/init.sls in an {% if %}.

Change-Id: I8260fdf5cf802c0b28197516da374add6c3002a2

This state enables the official saltstack package repository in order to
always benefit from the latest version. This state currently only works on
Debian and Ubuntu, and aims to implement the installation recommendations
of the official documentation:
http://docs.saltstack.com/en/latest/topics/installation/index.html