Upgraded Apache Ant

Turns out there was a security advisory posted on Ant 1.8.3
(http://ant.apache.org/security.html) though the affected code is not
used in Jenkins.

Reference: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2012-2098
Integrated the new version.

This version includes a change from christ66 in
https://github.com/stapler/stapler/pull/40