Fix sourcing brew's autojump of version 21.0.3+

Starting with autojump 21.0.3+ there is no file autojump in etc/
directory, so we source the autojump.zsh file directly. Version 20 has
it as well, so there should be no problems with earlier versions.

Comments on autojump's update in homebrew:
https://github.com/mxcl/homebrew/commit/5da9c4d5eb38e6abb516ba648898fb374ec360cd#Library/Formula/autojump.rb