Use dpkg_package resource to install mod_pagespeed

Using the ```package``` resource will default to ```apt``` and will error out with
```[2014-09-09T18:05:04+00:00] ERROR: package[mod_pagespeed]
(apache2::mod_pagespeed line 27) had an error:
Chef::Exceptions::Package: apt package provider cannot handle source
attribute. Use dpkg provider instead```