requirements.txt: pin swift 1.13.1

With the release of swift 2.0.0, some tests in ZeroCloud broke.

This commit
https://github.com/openstack/swift/commit/3824ff3df71975194fa1b050255c0da8c1acc598
took away the `object_ring` kwarg from the
swift.proxy.server.Application class, which the ZeroCloud tests were
using.

Until we can update everything to be compatible with swift 2.0, we can
stay at version 1.13.1.