Make compatible with chef 11

As of chef 11, "shef" became "chef-shell". I updated this to use
Shell::Extensions. These changes work for me when auditing my own
chef 11 server. I'm not sure if anything else needs to change.

I bumped the version from 0.2.0 to 1.0.0 per semver: this change is
non-backwards compatible.
Merge pull request #3 from groknaut/master

Make compatible with chef 11.  Many thanks!