Remove gnu vs osx specific sed buffer option

Hi Tim

I use gnu sed by default on osx, and google tells me I'm not the only one. I had to make the change in this patch to get vim-dispatch to work. Without this it fails silently (the error file is created empty).

I tried seeing if there was a reliable way to distinguish which sed flavour is installed, but failed to find a simple way. 

I'm not sure whether the sed buffering setting is really necessary so maybe the simplest is to get rid of it? Seems to work fine.

Cheers
Kris