Suggested gm overwrite

This admittedly evil overwrite fixes the fatal error that gm throws (it happens at `gm.write()` by the way) - for me. I do not know if any functionality is lost, because `npm test` does not work properly on my machine. I did compare `master` with my branch for my local Chrome, Firefox and PhantomJS and I only gained functionality. 

Please review and consider merging. Being able to make screenshots is quite a central feature to using selenium - at least for me.