Install a recent TracSpamPlugin and spambayes

The exact versions for these are based on the Django trac installation, which has a well working spam filter: https://github.com/django/code.djangoproject.com/blob/master/requirements.txt
Prefer HTTPS and add an explanatory comment.
Moved installation of spam filtering dependencies to the update command
Different domain
Merge pull request #17 from alex/patch-1

Install a recent TracSpamPlugin and spambayes