chore(requirements): update django-auth-ldap to 1.2.15

See https://django-auth-ldap.readthedocs.io/en/stable/changes.html#v1-2-15-2017-08-17-documentation-improvements

Author: mboersma

rootfs/requirements.txt

@@ -1,7 +1,7 @@
 # Deis controller requirements
 backoff==1.4.3
 Django==1.11.4
-django-auth-ldap==1.2.14
+django-auth-ldap==1.2.15
 django-cors-middleware==1.3.1
 django-guardian==1.4.9
 djangorestframework==3.6.4