upgrade javassist in order to make unit test (DubboSerializationTest) pass under java7/8

pom.xml

@@ -82,7 +82,7 @@
 	<properties>
 		<!-- Common libs -->
 		<spring_version>2.5.6.SEC03</spring_version>
-		<javassist_version>3.15.0-GA</javassist_version>
+		<javassist_version>3.20.0-GA</javassist_version>
 		<netty_version>3.2.5.Final</netty_version>
 		<mina_version>1.1.7</mina_version>
 		<grizzly_version>2.1.4</grizzly_version>