Problem with using Promises in TypeScript

The SDK has some asynchronous methods that are supposed to support using either callbacks or Promises:

```
client.variation(key, user, default, (err, result) => { ... }); // callback
client.variation(key, user, default).then(result => { ... }); // Promise
```

However, due to a problem in the SDK's TypeScript declarations, the second line above does not compile in TypeScript.