Bump to version 0.8.4

Author: hsluoyz

setup.py

@@ -7,7 +7,7 @@
 
 setuptools.setup(
     name="casbin",
-    version="0.8.3",
+    version="0.8.4",
     author="TechLee",
     author_email="techlee@qq.com",
     description="An authorization library that supports access control models like ACL, RBAC, ABAC in Python",