Feature/atomic repo optional

I needed to install PHP 5.5 on a CentOS box, but unfortunately this cookbook automatically installs the `atomic` repo on RHEL systems - which doesn't seem to support PHP 5.5 yet.

In addition, the `yumrepo` cookbook hasn't been updated for version 3.0 of the `yum` cookbook (which my other cookbooks require).

So I think the best solution is to just make usage of the atomic repo optional and default it to *yes*, so it doesn't break any backwards compatibility.

This is fully tested with CentOS 6.5 via Vagrant.