Updates for chef 12

Fixes #15 

This seems to be the set of changes needed for the cookbook to work on Chef 12. 

Chef 12's RFC is [on the chef-rfc repo](https://github.com/opscode/chef-rfc/blob/70d848f45b0f34d201585abc1740f1a6f85ba616/rfc015-chef-12.md). Despite the statement there, it seems that Chef 12 shipped with Ruby 2.1.4p265 (not 2.1.2). 

The thing we were primarily running into with #15 however was [the changes around how provider resolution is performed](https://github.com/opscode/chef/blob/12.0.0/RELEASE_NOTES.md#resource-and-provider-resolution-changes). Chef deprecated the existing massive map binding in favor of the ProviderResolver (to get around a certain class of resolution issues). As a result we were no longer binding to the Template provider for our resource, but by adding that binding explicitly we get around that issue.
Because Ruby is very flexible, the constant Chef::ProviderResolver may not be a Class. I found it on [this stackoverflow answer](http://stackoverflow.com/a/1187152/504685)