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.
If you are ensuring Chef::ProviderResolver is defined, what does checking the class add?