update to new version of django-babel-underscore, which supports Django 1.8 - Django 1.11

requirements/edx/base.txt

@@ -14,7 +14,7 @@ cryptography==1.5.3
 cssselect==0.9.1
 dealer==2.0.4
 defusedxml==0.4.1
-django-babel-underscore==0.5.1
+django-babel-underscore==0.5.2
 markey==0.8  # From django-babel-underscore
 django-config-models==0.1.8
 django-countries==4.0