Point to GH issues in README instead of LightHouse

Author: solnic

README.rdoc

@@ -5,7 +5,7 @@
 DataMapper sports a very accessible code-base and a welcoming community.
 Outside contributions and feedback are welcome and encouraged, especially
 constructive criticism. Make your voice heard! Submit a
-ticket[http://datamapper.lighthouseapp.com] or
+ticket[https://github.com/datamapper/dm-core/issues] or
 patch[http://datamapper.lighthouseapp.com], speak up
 on our mailing-list[http://groups.google.com/group/datamapper/], chat with us
 on irc[irc://irc.freenode.net/#datamapper], write a spec, get it reviewed, ask