ts error in index.d.ts

Hi, I have a TS error in `index.d.ts`:

`Construct signature, which lacks return-type annotation, implicitly has an 'any' return type.`

Indeed, there's a missing return type in line 12 (for the `new ()` construct):
```typescript
export function Mixins<T>(...components: { new (); }[]): VirtualClass<T>
```
