Multiple super classes got stripped

**What version of TypeScript are you using?**
2.9.1

**What version of `typescript-eslint-parser` are you using?**
16.0.0

**What code were you trying to parse?**

```ts
class X extends A, B {}
```

**What did you expect to happen?**
An error.

**What happened?**
Multiple super classes got stripped.

```ts
class X extends A {}
```

