Add benjamingr to collaborator list.
Related https://github.com/nodejs/node/issues/5064
PR-URL: https://github.com/nodejs/node/pull/5664
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
### Collaborators
+* [benjamingr](https://github.com/benjamingr) - **Benjamin Gruenbaum** <benjamingr@gmail.com>
* [brendanashworth](https://github.com/brendanashworth) - **Brendan Ashworth** <brendan.ashworth@me.com>
* [calvinmetcalf](https://github.com/calvinmetcalf) - **Calvin Metcalf** <calvin.metcalf@gmail.com>
* [domenic](https://github.com/domenic) - **Domenic Denicola** <d@domenic.me>