increment version to 4.0.20

Author: michaelsembwever

build.xml

@@ -34,7 +34,7 @@
     <property name="debuglevel" value="source,lines,vars"/>
 
     <!-- default version and SCM information -->
-    <property name="base.version" value="4.0.19"/>
+    <property name="base.version" value="4.0.20"/>
     <property name="scm.connection" value="scm:https://gitbox.apache.org/repos/asf/cassandra.git"/>
     <property name="scm.developerConnection" value="scm:https://gitbox.apache.org/repos/asf/cassandra.git"/>
     <property name="scm.url" value="https://gitbox.apache.org/repos/asf?p=cassandra.git"/>