Silence NameError when deleting ImproperlyConfigured

Closes #30.

bigbluebutton/api/_caching.py

@@ -11,7 +11,11 @@ else:
 
     USING_DJANGO = True
 finally:
-    del ImproperlyConfigured
+    try:
+        del ImproperlyConfigured
+    except NameError:
+        # We never got it
+        pass  # noqa
 
 try:
     # Try importings ucache's most simple backend