Update Trac instructions. Fix #794.

This commit doesn't touch trac.rb to avoid breaking existing installations of http://github.com/davglass/github-trac. trac-github is recommended for new installations.
Clarify that the Trac service is deprecated
Webhook is a non-proper noun.
Merge pull request #795 from aaugustin/master

Update Trac instructions. Fix #794.