fixed a derp in promisify

Author: Salakar

lib/utils/promisify.js

@@ -9,6 +9,6 @@ export default(fn, NativeModule) => (...args) => {
   return new Promise((resolve, reject) => {
     const _fn = typeof fn === 'function' ? fn : NativeModule[fn];
     if (!_fn || typeof _fn !== 'function') return reject(new Error('Missing function for promisify.'));
-    return fn.apply(NativeModule, ...args, handler.bind(handler, resolve, reject));
+    return _fn.apply(NativeModule, ...args, handler.bind(handler, resolve, reject));
   });
 };