Updating xstream due security issues and other improvements

As described in release notes:

> This maintenance release addresses mainly the security vulnerability CVE-2013-7285, an arbitrary execution of commands when unmarshalling.

And with 1.4.6 works fine under GAE/J environment. May be useful in the future when vraptor4 can be able to run under GAE. Issues XSTR-566 and XSTR-200.