Do not crash if non-prefixed AudioContext isn't available

[warpdesign]

Safari 11.x only has prefixed webAudioContext: without this patch the polyfill crashes.

Fixes #9

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[warpdesign]

I signed it.

[googlebot]

CLAs look good, thanks!

[developit]

just a small change - I can make it if you want, or you can!

[developit]

this will crash if neither are defined. Need to do:

(self.AudioContext || self.webkitAudioContext)

[developit]

FWIW I usually recommend "polyfilling" AudioContext in Safari globally:

if (!window.AudioContext && window.webkitAudioContext) = window.AudioContext = window.webkitAudioContext;

[developit]

Shipped in 1.1.1! Cheers.

[warpdesign]

My app now work perfectly in Safari & Firefox!