Bump up the version to 0.4.0 (#81)

Author: kezhenxu94

setup.py

@@ -25,7 +25,7 @@
 
 setup(
     name="apache-skywalking",
-    version="0.3.0",
+    version="0.4.0",
     description="Python Agent for Apache SkyWalking",
     long_description=README,
     long_description_content_type="text/markdown",