Use git to download and install chruby

Use git to download the chruby repo (from any (forked) repo) and install.

Downside to this is that we can't verify the download anymore, but that wasn't be done anyway.

Another approach would be to use [remote_file](http://docs.opscode.com/resource_remote_file.html), link to the direct `https://codeload.github.com/postmodern/chruby/tar.gz/v#{node['chruby']['version']}` download and install like the nginx cookbook does.
See downloading [here](https://github.com/opscode-cookbooks/nginx/blob/master/recipes/source.rb#L58), extracting [here](https://github.com/opscode-cookbooks/nginx/blob/master/recipes/source.rb#L82) and installing [here](https://github.com/opscode-cookbooks/nginx/blob/master/recipes/source.rb#L97).

Downside to this is not being able to select specific non-released version of chruby, although I doubt one should really be able to install them, they are not released for a reason.

Fixes #19