Ruby 2.0

This seems too easy - am I missing something? [This comment](http://chocolatey.org/packages/ruby#comment-790703717) seems to suggest there needs to be an uninstall script now - I guess that might be it.

TODO:
 
 - [x] use x64 build for 2.0.0-p195 if on 64-bit Windows
 - [x] back out the changes to drop the old versions 
 - [ ] test uninstall behaviour and confirm nothing custom hangs around

And of course `ruby.devkit` has some new releases which are 32/64-bit dependent too.