Check for wheel install before allowing setup.py publish.

setup.py

@@ -59,6 +59,9 @@ version = get_version('rest_framework')
 
 
 if sys.argv[-1] == 'publish':
+    if os.system("pip freeze | grep wheel"):
+        print("wheel not installed.\nUse `pip install wheel`.\nExiting.")
+        sys.exit()
     os.system("python setup.py sdist upload")
     os.system("python setup.py bdist_wheel upload")
     print("You probably want to also tag the version now:")