fix WebOb dependency

tools/pip-requires list WebOb in lower case which confuse pip freeze and
gives a false positive:

Before patch:

 $ pip freeze -r tools/pip-requires 2>&1|grep -i webob
 Requirement file contains webob, but that package is not installed
 WebOb==1.1.1
 $

The second line comes from another packages which list 'WebOb' as a
dependency.

After patch:

 $ pip freeze -r tools/pip-requires 2>&1|grep -i webob
 WebOb==1.1.1
 $

Change-Id: I73d01c734c25fc65022de146264e5b2c3cc2e67f
Reviewed-on: https://review.openstack.org/26786
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins