Fix up things for final Chef 12 release

- Check the run_context in the chef_run when rendering
- Move `save` call into the ServerRunner
- Add Chef 12 to the test matrix
- Drop support for Ruby 1.9.3
- Fix a recursive require bug in Chef core by requiring chef/config first
Meta-yield
Merge pull request #545 from sethvargo/sethvargo/12

Minor 12 fixes