Disable request pooling

The underlying implementation is unusably buggy — see
https://github.com/mikeal/request/issues/465 — which means that
processing more than a small number of pages (I found somewhere
around 15) will cause ucss to hang with no obvious cause.

Since the actual server response times are almost instantaneous this is
presumably a bug somewhere else (apparently redirect handling); setting
a timeout will cause all 5 (i.e. http.globalAgent.maxSockets) requests
to terminate simultaneously but simply disabling pooling altogether
avoids the problem entirely and makes total execution faster with ~50
total URLs.