Ensure the rubies directory exists

I finally got a chance to give this a shot!

I ran into a problem where it failed and complained that the `/opt/boxen/chruby/opt/rubies` directory didn't exist.

I tried to fix this with a `file` resource, but it turns out Puppet won't recursively create directories in a path, so I had to shell out to `mkdir -p`.

This works, but it's probably terrible Puppet. Feel free to laugh and criticise!