autojump: install completion into site-functions

`_autojump` needs to be installed into `share/zsh/site-functions`
not `share/zsh/functions`.

Files in `share/zsh/functions` must not change unless zsh is
upgraded and zsh is restarted.  `site-functions` can change during a zsh
process's lifetime.

This will enable homebrew's zsh to automatically use the completion.
See issue Homebrew#11936 for details.

Closes Homebrew#11942.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Formula/autojump.rb

@@ -14,7 +14,7 @@ def install
     man1.install 'autojump.1'
     (prefix+'etc').install 'autojump.sh' => 'autojump'
     (prefix+'etc').install 'autojump.bash', 'autojump.zsh'
-    (share+'zsh/functions').install '_j'
+    (share+'zsh/site-functions').install '_j'
   end
 
   def caveats; <<-EOS.undent
@@ -23,6 +23,9 @@ def caveats; <<-EOS.undent
     if [ -f `brew --prefix`/etc/autojump ]; then
       . `brew --prefix`/etc/autojump
     fi
+
+    zsh completion has been installed to:
+      #{HOMEBREW_PREFIX}/share/zsh/site-functions
     EOS
   end
 end