Replaces webkitAudioContext.createGainNode() with createGain()

Game would not load in Chrome v38 due to bug in main.coffee line 33: mixer = audioCtx?.createGainNode()
webkitAudioContext.createGainNode() was deprecated and replaced with createGain() as per the latest W3C Web Audio API spec - it appears that later versions of Chrome no longer support the former.