Fix gate requirement checks job

Current requirements-check job is failing with
below error:

ERROR: Requirement for package PrettyTable excludes a version not excluded in the global list.
  Local settings : {'<0.8'}
  Global settings: set()
  Unexpected     : set()
Validating test-requirements.txt

Keeping PrettyTable same as what we have in openstack/requirements repo

Change-Id: I63633d2932757ca23bcea69fd655a2499a5b6d31

requirements.txt

@@ -28,7 +28,7 @@ oslo.versionedobjects>=1.32.0 # Apache-2.0
 PasteDeploy>=1.5.2 # MIT
 pbr>=3.1.1 # Apache-2.0
 pecan>=1.3.2 # BSD
-PrettyTable<0.8,>=0.7.2 # BSD
+PrettyTable>=0.7.2 # BSD
 gnocchiclient>=7.0.1 # Apache-2.0
 python-ceilometerclient>=2.9.0 # Apache-2.0
 python-cinderclient>=3.5.0 # Apache-2.0