Use a seemingly more maintained fork of xapian-full

Hi,

I just found in the sup-devel mailing list that this project is still maintained here. I'm a frustrated Thunderbird user and webmail is just not my thing. I'd really like to see project sup come back in good shape and become a user(or even better, a committer).

Doing `gem i xapian-full` on my Archlinux system just fails with this log: https://gist.github.com/5long/5213393 This patch makes sup uses a fork of xapian-full which works.

To be clear, this change alone doesn't make sup gem-i-installable for me. I just got stuck on another error when doing `rake gem`. But this patch is worth reviewing on its own, I guess.