Fix gate to add babel in requirement

Current gate is failing with below error
-   ModuleNotFoundError: No module named 'babel'

Change-Id: If701a694dff4189256e28afac061f4abc8fb8fbf

lower-constraints.txt

@@ -1,3 +1,4 @@
+Babel==2.3.4
 coverage==4.5.2
 elasticsearch==2.0.0
 eventlet==0.18.2

requirements.txt

@@ -9,6 +9,8 @@ pbr!=2.1.0,>=2.0.0 # Apache-2.0
 # package to get the right headers...
 greenlet>=0.4.10 # MIT
 
+Babel!=2.4.0,>=2.3.4 # BSD
+
 eventlet!=0.18.3,!=0.20.1,>=0.18.2 # MIT
 PasteDeploy>=1.5.0 # MIT
 Routes>=2.3.1 # MIT