Async not used in Vim

[Sharlock93]

Async searching is not used in newer versions, the check should be changed to has('job') rather than a specific patch.

ferret/autoload/ferret/private.vim

Line 83 in 3d06430

return l:async && has('patch-7-4-1829')

[wincent]

Thanks for the report, and the suggested fix, @Sharlock93!