Modify required Django version to match django-oscar requirements

W/o this change for tests will be installed the last version of Django
(2.2) and we will get the next error: Django 2.2 is installed but
django<2.2,>=1.11 is required by {'django-oscar'}.

setup.py

@@ -33,7 +33,7 @@
     install_requires=[
         'phonenumbers',
         'pillow',
-        'django>=1.11',
+        'django>=1.11,<2.2',
         'django-oscar>=1.6',
         'django-phonenumber-field>=2.0,<2.1',
     ],