Switch xapian binding to xapian-ruby

Another therapeutic patch for myself.

This is a follow up of #60. I've been using xapian-ruby=1.2.15.1 locally for a while without visible problems. But if a user has both xapian-ruby and xapian-full-alaveteli installed, only xapian-full-alaveteli would be activated when doing `require "xapian"`. Thus 	aa7691d is needed.

Besides, I've found that I'd been using `~>` in a wrong way. Fixed in 4c94418.