Async function expressions are marked as not async

**What version of TypeScript are you using?**

2.3.2

**What version of `typescript-eslint-parser` are you using?**

`master`

**What code were you trying to parse?**

```
(async function test() {})();
```

**What did you expect to happen?**

`async: true` in `FunctionExpression` node.

**What happened?**

`async: false` in `FunctionExpression` node.

Regression from #229
