Rename LICENCE to LICENSE

[AaronBaker]

When installing AMSmoothAlert using CocoaPods, I got the following error:

[!] Unable to read the license file /Users/aaron/Documents/YumList/Pods/AMSmoothAlert/LICENSE for the spec AMSmoothAlert (1.1)

I believe this error is caused by using the british spelling for the word "LICENSE"

[chrismaddern]

It'd be great if you can merge and release this :)

It's the only pod warning we have :dance:

[chrismaddern]

But also.. the British spelling is ofcourse right :D