upgraded to latests XStream version -- have tested and it seems to wo…

…rk, but moving above 1.4.17 means that we now have to use our own security instance. We already do this in core which means everything here should be fine, but any plugins which use XStream may have an issue. If so then they just need to call Gate.configureXStreamSecurity(XStream) prior to using the instance to have the same configuration applied

pom.xml

@@ -399,7 +399,7 @@
 		<dependency>
 			<groupId>com.thoughtworks.xstream</groupId>
 			<artifactId>xstream</artifactId>
-			<version>1.4.17</version>
+			<version>1.4.20</version>
 			<scope>compile</scope>
 			<exclusions>
 				<!-- we will pull in the full xpp3 so we don't need these -->