Specifies gabbi version in test-requirements.txt
Specifies gabbi version in test-requirements.txt

Because of the modification of the handlers module in gabbi 1.26,
the python2.7 gate jobs did not work properly. This patch specifies that
the gabbi version must be inferior to 1.26.

Change-Id: I69f93959f30a829d746da24a17c10c8f75d60d6a
Closes-Bug: 1628930

openstack/cloudkitty,openstack/cloudkitty,stackforge/cloudkitty,stackforge/cloudkitty