Start xfvb directly on Travis, customise resolution

This should fix specs that only fail on Travis, or when run headless.

See http://docs.travis-ci.com/user/gui-and-headless-browsers/#Configuring-xvfb-screen-size-and-more

/ HT @hschink

Signed-off-by: Alex Coles <alex@alexbcoles.com>
xvfb on Travis: adjust pidfile path
xvfb on Travis: unquieten start-stop-daemon
xvfb on Travis: show additional debug info