Use handlers with Dispatch, not plain copen

Otherwise e.g. when you have vertical splits open, the quickfix list
will not span the full bottom of the Vim window like standard ack.vim
behavior. Also broke location list entirely...

autoload/ack.vim

@@ -53,7 +53,7 @@ function! ack#Ack(cmd, args)
   if !g:ack_use_dispatch
     call ack#show_results()
   else
-    copen
+    execute s:handler
   endif
   call <SID>apply_maps()
   call <SID>highlight(l:grepargs)

plugin/ack.vim

@@ -22,7 +22,13 @@ if !exists("g:ack_apply_lmappings")
   let g:ack_apply_lmappings = !exists("g:ack_lhandler")
 endif
 
-if !exists("g:ack_use_dispatch")
+if exists("g:ack_use_dispatch")
+  if g:ack_use_dispatch && !exists(":Dispatch")
+    let msg = "Ack: Dispatch not loaded! Falling back to g:ack_use_dispatch = 0."
+    echohl WarningMsg | echom msg | echohl None
+    let g:ack_use_dispatch = 0
+  endif
+else
   let g:ack_use_dispatch = 0
 end