deal with the insanity of amazon linux
added logic to not attempt updates on debian 5 based system
Update conditional to use `Gem::Version` instances for version comparison
Debian versions need to be major-only.
Initial run at code to prevent downgrade of chef by the cookbook.

* Fixed logic to ensure that always_download continues to behave the same way, and remove extraneous bits from version string.
* Updated README.md to note prevent_downgrade attribute