requirements: Upgrade django-auth-ldap to 1.6.0.

The autenticate function now follows the signature of
Django 2.0 https://github.com/django-auth-ldap/
django-auth-ldap/commit/27a8052b26f1d3a43cdbcdfc8e7dc0322580adae

Also AUTH_LDAP_CACHE_GROUPS is depricated in favor of
AUTH_LDAP_CACHE_TIMEOUT.

zproject/backends.py

@@ -292,6 +292,7 @@ class ZulipLDAPAuthBackend(ZulipLDAPAuthBackendBase):
             return None
 
         return ZulipLDAPAuthBackendBase.authenticate(self,
+                                                     request=None,
                                                      username=username,
                                                      password=password)