Add support for Buffer

Follow up to https://github.com/prerender/prerender-node/pull/15, which didn't actually add support for Buffer since they've stopped using the BufferBot user agent and switched to a custom header.

From [Buffer](https://bufferapp.com/):
>So as far as i'm aware we fake our agent to be a real browser as some sites return 'incompatible browser' otherwise. We do have a HTTP header which you can look for which is... "X-BufferBot: Being Awesome! P.S. We're hiring! jobs.bufferapp.com"