set nofile limits in Polipo plist file (default 256 is really small)

Homebrew `polipo` bottle includes a `homebrew.mxcl.polipo.plist` file to launch Polipo on boot.

I had problems with this file due to small NOFILE limit (that is, by default Polipo is able to open 256 file or network streams at one time, which is a small value).

I highly suggest raising this limit. I'm using 10240 (as in the provided commit) and yet have no problems with Polipo.