Bump up the version to 0.4.0 (#81)

diff --git a/setup.py b/setup.py
index b2e2b0b..276cf81 100644
--- a/setup.py
+++ b/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",