Add support for Chef 11

This PR is dependent on opscode/chef#623 (Relax version constraint on JSON).

:warning: Needs review!

At the high level, it adds support for Chef 11.

- Monkey Patch both `Chef::Provider` and `Chef::Provider::LWRPBase` (in Chef 11)
- Conditional tests (based on `Chef::VERSION`)
- Cleans up gemspec a bit
- Removes tests (but not support) for Chef 9. Chef 9 was officially deprecated in 2012.

:warning: Needs review!