Add an explicit --nopager to default ack options

Otherwise if a user has something like `--pager=less` as a default in
their `~/.ackrc`, Ack may think that it's running interactively if
executed in a separate shell e.g. when `g:ack_use_dispatch` is enabled.
This will result in broken behavior loading results in Vim.

See #182 for some background.

plugin/ack.vim

@@ -3,7 +3,7 @@ if exists('g:loaded_ack')
 endif
 
 if !exists("g:ack_default_options")
-  let g:ack_default_options = " -s -H --nocolor --nogroup --column"
+  let g:ack_default_options = " -s -H --nopager --nocolor --nogroup --column"
 endif
 
 " Location of the ack utility