Use different function syntax that the JS parser used to build the website can understand

Summary:
For some reason the parser doesn't support that syntax. It fails with:

```
Cannot parse file ../Libraries/Animated/src/AnimatedImplementation.js Error: parseSource returned falsy
    at parseAPIInferred (/home/ubuntu/react-native/website/server/extractDocs.js:343:13)
```

Reviewed By: sahrens

Differential Revision: D4669961

fbshipit-source-id: 3536c94b8f385f5810c47544c4a72c8c785bf0e5

Libraries/Animated/src/AnimatedImplementation.js

@@ -2215,6 +2215,7 @@ function unforkEvent(event: ?AnimatedEvent | ?Function, listener: Function): voi
 class AnimatedEvent {
   _argMapping: Array<?Mapping>;
   _listeners: Array<Function> = [];
+  _callListeners: Function;
   _attachedEvent: ?{
     detach: () => void,
   };
@@ -2228,6 +2229,7 @@ class AnimatedEvent {
     if (config.listener) {
       this.__addListener(config.listener);
     }
+    this._callListeners = this._callListeners.bind(this);
     this._attachedEvent = null;
     this.__isNative = shouldUseNativeDriver(config);
 
@@ -2281,9 +2283,9 @@ class AnimatedEvent {
     };
   }
 
-  _callListeners = (...args) => {
+  _callListeners(...args) {
     this._listeners.forEach(listener => listener(...args));
-  };
+  }
 
   _validateMapping() {
     const traverse = (recMapping, recEvt, key) => {