make current async version compulsory

I've been having intermittent problems with my code, giving the error message `TypeError: Object #<Object> has no method 'each'`. I've traced the problem to `express-form`, specifically when the version of the `async` requirment which gets installed is `async@0.1.0`. The problem isn't visible when the version which gets installed is `async@0.2.9`.

The existing `~` in the `async` dependency in `package.json` lets the earlier version of the package be installed.

For reference:
```
$ npm --version
1.3.0
$ node --version
v0.8.25
```