Replace appspot URLs with github

[nponeccop]

There are 6 places which list the old homepage:

[foo@bar ShipIt-0.60]$ grep -R appspot .
./README:    http://contributing.appspot.com/shipit
./shipit:L<http://contributing.appspot.com/shipit>
./CONTRIBUTING:   http://contributing.appspot.com/shipit
./ChangeLog:    * Added a bunch of links to http://contributing.appspot.com/shipit
./lib/ShipIt.pm:L<http://contributing.appspot.com/shipit>
./AUTHORS:# Got a patch?  See: http://contributing.appspot.com/shipit

Actually 5, because ChangeLog doesn't count.