Remove Ryu from the global dependency

This patch removes Ryu from requirements.txt because Neutron community
started thinking about to introduce the agent specific package
resolution mechanism. See the following link for more details:

https://bugs.launchpad.net/neutron/+bug/1033186

Change-Id: I5a538cd9211f3aad23d9129dcd7cc492b0d64894
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>