MAINT allow setting cache dir without setting a private cache dir

Author: mfeurer

openml/config.py

@@ -70,6 +70,9 @@ def set_cache_directory(cachedir, privatedir):
     get_cache_directory
     get_private_directory
     """
+	if privatedir is None:
+        privatedir = cachedir
+
     global _cachedir
     global _privatedir
     _cachedir = cachedir