Update es6-destructuring to v0.3.1.

This fixes a bug where destructuring assignment was not handled inside
variable declaration nodes, which in particular affected functions and
class bodies, e.g.

```js
var foo = function() {
  // destructuring in here did not work.
};

class foo {
  constructor() {
    // destructuring in here did not work.
  }

  a() {
    // nor here.
  }
}
```
v0.11.1