add fix for RN v0.31

Author: JakeRawr

ios/RCTWebViewBridge.m

@@ -21,6 +21,7 @@
 #import "RCTUtils.h"
 #import "RCTView.h"
 #import "UIView+React.h"
+#import <objc/runtime.h>
 
 //This is a very elegent way of defining multiline string in objective-c.
 //source: http://stackoverflow.com/a/23387659/828487