fix RuntimeWarning during imp_load in custom settings file (#185)

Author: danielhochman

pynamodb/settings.py

@@ -19,7 +19,7 @@
 
 override_settings = {}
 if os.path.isfile(OVERRIDE_SETTINGS_PATH):
-    override_settings = imp.load_source(OVERRIDE_SETTINGS_PATH, OVERRIDE_SETTINGS_PATH)
+    override_settings = imp.load_source('__pynamodb_override_settings__', OVERRIDE_SETTINGS_PATH)
     log.info('Override settings for pynamo available {0}'.format(OVERRIDE_SETTINGS_PATH))
 else:
     log.info('Override settings for pynamo not available {0}'.format(OVERRIDE_SETTINGS_PATH))