Update to the latest stevedore

Use a version of stevedore that does not try to
install distribute.

The requirements change is in
https://review.openstack.org/#/c/32738/

Change-Id: I727dd6b4c8d8bab271c51c23bba6945cf5cd6225
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>