We should require a minimum version of ant for the installer

We should require a minimum version of ant for running the installer - it would cut down on our test matrix. dward is seeing :
Running the installer (from the expanded target zip):
(02:02:54 PM) errantepiphany: [echo] Unzipped SwitchYard forge plugin.
(02:02:55 PM) errantepiphany: [echo] Installing forge plugin to /home/errantepiphany/.forge ...
(02:02:55 PM) errantepiphany: [java] javax.xml.parsers.FactoryConfigurationError: Provider org.apache.xerces.jaxp.DocumentBuilderFactoryImpl not found
(02:03:21 PM) errantepiphany: tcunning: java version "1.6.0_29"

Using ant 1.7.1. We may want to require ant 1.8.1 for running the script.