Adds missing WebOb in requirements.txt

Unit tests are failing without it, and the file
oslo_versionedobjects/exception.py has:
import webob.exc

So it's looking like webob is missing from requirements.txt.
This patch fixes that. Note that this should be fixed in
the kilo branch later on.

Change-Id: Ib0ac48aeea4f8413740af89340e3b094312fa6b4
(cherry picked from commit c7ff6a07cc)

requirements.txt

@@ -13,3 +13,4 @@ oslo.log>=1.0.0,<1.1.0  # Apache-2.0
 oslo.i18n>=1.5.0,<1.6.0  # Apache-2.0
 mock>=1.0
 fixtures>=0.3.14
+WebOb>=1.2.3