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.
Merge pull request #96 from larsbutler/pin-swift-to-1.13.1

requirements.txt: pin swift 1.13.1