update jackrabbit webdav lib to version 2.6.4

We used an old version of jackrabbit. Since 2.2.5 there were many updates/bugfixes/securitiy issues fixed. Cf. http://www.apache.org/dist/jackrabbit/2.6.4/RELEASE-NOTES.txt

And a nice side effect: ownCloud.apk is well 100kb smaller now!