Async function expressions are marked as not async

[azz]

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