Fix memory leak issue

The ```requires_arc``` in podspec file makes the Amplitude SDK from cocoapods could be compiled with ARC support. (It doesn't mean whether these source code files requring ARC or not. It means to enable ARC or not)

The main change in ```Amplitude.m``` is near line 889.
The ```uuidStr``` which is returned by ```CFUUIDCreateString``` should be released before returning the method. (```-stringByAppendingString:``` returns another instance of NSString which is put into autorelease pool already, Hence the ```uuidStr``` is unused and should be returned.)