Added dealloc call to super which removes all observers to self

CDVPlugin does some work in its dealloc - https://github.com/apache/cordova-ios/blob/master/CordovaLib/Classes/CDVPlugin.m

If super is not called, I don't think these will get unobserved - https://github.com/apache/cordova-ios/blob/master/CordovaLib/Classes/CDVPlugin.m#L48

Not a Cordova expert yet but let me know if I'm mistaken :)