Fix ackpreview when there are splits

When previewing we want to open the result and then move back to the
results buffer, the previous window (`:h CTRL-W_p`) -- not cycle through
windows (`:h CTRL-W_w`) which would include other splits besides the
results list.

Fixes #238

autoload/ack.vim

@@ -130,8 +130,8 @@ function! s:ApplyMappings() "{{{
   endif
 
   if exists("g:ackpreview") " if auto preview in on, remap j and k keys
-    nnoremap <buffer> <silent> j j<CR><C-W><C-W>
-    nnoremap <buffer> <silent> k k<CR><C-W><C-W>
+    nnoremap <buffer> <silent> j j<CR><C-W><C-P>
+    nnoremap <buffer> <silent> k k<CR><C-W><C-P>
     nmap <buffer> <silent> <Down> j
     nmap <buffer> <silent> <Up> k
   endif