Set maximum version of supported DRF and Django (#808)

Avoid failling of master if we haven't added support
for newly released versions yet.

Author: sliverc

setup.py

@@ -90,8 +90,8 @@ def get_package_data(package):
     ],
     install_requires=[
         'inflection>=0.3.0',
-        'djangorestframework>=3.10',
-        'django>=1.11',
+        'djangorestframework>=3.10,<3.12',
+        'django>=1.11,<3.1',
     ],
     extras_require={
         'django-polymorphic': ['django-polymorphic>=2.0'],