Fix compatibility with gabbi >= 2.7.0

Fix the argument error caused by the change[1] in gabbi 2.7.0 .

[1] https://github.com/cdent/gabbi/commit/8511156559651ab0bf9d2679816d5a7818af290b

Change-Id: I936a3e6ddb0c254069839aaaa1f0eb7d4a72c85d
diff --git a/requirements.txt b/requirements.txt
index 4e6bb28..17f694d 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,9 +1,5 @@
-# The order of packages is significant, because pip processes them in the order
-# of appearance. Changing the order has an impact on the overall integration
-# process, which may cause wedges in the gate later.
-
 pbr>=2.0 # Apache-2.0
 oslo.config>=6.0.0 # Apache-2.0
 oslo.utils>=3.37.0 # Apache-2.0
 tempest>=17.1.0 # Apache-2.0
-gabbi>=1.30.0 # Apache-2.0
+gabbi>=2.7.0 # Apache-2.0