fix for new backend changes. Pass forward slash on Unix

I made a change to the server that requires that you pass in the native separator (depending on platform) rather than backslashes.

This change seems to work for me, but I no nothing about elisp or your code base :) 

Sorry about this... I only just realised that omnisharp-emacs was broken against the latest server. Until this is merged, you will need to use server at SHA 512f7666fa.