Update WebOb version to >=1.2

The other projects have merged changes to accept
WebOb>=1.2 now, so update Quantum to use the same
version.

See bug #1092227 for details.

Change-Id: I0969752910b92209408d956f633ad2ccacc7fa46
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>