Added support for ruby binaries

Binaries are currently loaded from intercityup.com/binaries/ but if you
want you can change that url by overriding
default['rbenv']['binaries_url']. Also you can add extra binaries that
you want to support to the defaul['rbenv']['available_binaries'] array.

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>
Merge pull request #75 from jvanbaarsen/use-ruby-binaries

Use ruby binaries