Add setter to FBSettings.AppVersion

From the [FB documentation](https://developers.facebook.com/docs/reference/ios/current/constants/FBSettings):

```
setAppVersion:
Sets the application version to the provided string. FBAppEvents, for instance, attaches the app version to events that it logs, which are then available in App Insights.

+ (void)setAppVersion:(NSString *)appVersion;
Parameter
appVersion
The version identifier of the iOS app.
```