Hopefully get auto-updates from GitHub working.

Hey Tom,

I've added a little bit of code to stop GitHub clobbering auto-updates. (There's a header returned that stops Chrome updating extensions and I had the plugin strip them out.)

I tested it still installs and no errors in background console. I couldn't see how to test it working though and I obviously couldn't test updates as it is necessarily hardcoded to your repo. Hopefully it'll work though!

Cheers, Dave.