[trac] set minimum karma of spambayes to -14

[philll]

Background

See #683
Adding features not yet included in Trac's spam filtering plugin means continously maintining them for updated etc, which we want to avoid. With the statistical assumption that all real spam was rated with 100% spamage, leading to -15 karma points, setting the minimum required karma to -14 should keep real spam recognized as such while no longer annoying people with false positive.

What to change

Set the minimum karma points for non-spam commits in Trac's spam filter to -14

[philll]

I did so via the admin interface but vagrant updating the issues1 instance would overwrite that again, so it still needs to be changes in the config file.