Node 6.6

With node 6.6 I get a "global.env.a is not defined" error when using the example from the frontpage:

``` js
var Parallel = require('paralleljs')
var p = new Parallel([1, 2, 3], {
  env: {
    a: 10
  }
});

// returns 10, 20, 30
p.map(function (d) {
  return d * global.env.a;
});
```
