Declare Django versions in install_requires

Pip's dependency resolver (used in pipenv, pip-compile, poetry, etc.) can use this to infer whether there's a verison collision in what it's being asked to install or not.

setup.py

@@ -82,7 +82,7 @@ setup(
     author_email='tom@tomchristie.com',  # SEE NOTE BELOW (*)
     packages=find_packages(exclude=['tests*']),
     include_package_data=True,
-    install_requires=[],
+    install_requires=["django>=1.11,<3.1"],
     python_requires=">=3.5",
     zip_safe=False,
     classifiers=[