remove notification observer to avoid iOS 8 crash

src/objective-c/GRPCClient/GRPCCall.m

@@ -648,6 +648,8 @@ const char *kCFStreamVarName = "grpc_cfstream";
 }
 
 - (void)dealloc {
+  [GRPCConnectivityMonitor unregisterObserver:self];
+  
   __block GRPCWrappedCall *wrappedCall = _wrappedCall;
   dispatch_async(_callQueue, ^{
     wrappedCall = nil;