Fix command for ag-dired-regexp

It used to work well, but now it somehow doesn't work if command executed asynchronously (with '&').
I've checked following two environments.

* Emacs 24.3.1, MacOSX 10.8.5, ag 0.22.0
* Emacs 24.3.1, Ubuntu 14.04,  ag 0.19.2

And one more, wrapping `dir` path with quotes for a path includes spaces.

Could you confirm and pull this?