Remove `chruby <ruby>` from profile.d if default attr is nil.

I've found a couple of use cases where I'd rather not have a `chuby` exec in `/etc/profile.d/chruby.sh`, namely each user having their own separate default Ruby version. This commit will simply skip the line in the `chruby.sh` script when `node['chruby']['default']` is `nil` or `false`.