increase request version

We were having socket hangup errors in phaxio `provisionNumber` when we upgraded to Node 0.8.21. I believe we were seeing the socket issue raised by a bug fixed in 0.8.20. Here's a good summary:

http://clock.co.uk/tech-blogs/preventing-http-raise-hangup-error-on-destroyed-socket-write-from-crashing-your-nodejs-server

Increasing request version for phaxio to the current one seemed to have resolved the issue for us.