Fixed "unhandled 'error' event" when using env variables in "npm start".

I had a really strange problem when using env variables within my "npm start" declarations.

Something like:

```json
{
    "scripts": {
         "start": {
             "foo.com": "NODE_ENV=production node app.js"
         }
     }
}
```

caused an "unhandled 'error' event". This patch fixes this problem.