RubySpec moved to ruby-doc subdomain

I've finally transferred the RubySpec wiki to a server hosted under the 
ruby-doc "spec" subdomain.

http://spec.ruby-doc.org/wiki (spec.ruby-doc.org redirects here, since 
in the future there may be other things we'd like to host under this domain)

Any links on the main ruby-doc site that are pointing at headius.com 
should be updated to appropriate locations under the new address. 
headius.com/rubyspec/* are all set to redirect to the spec.ruby-doc.org 
site.

- Charlie