Update minimum Ant version. It must be 1.9.0 or later for Java 8 support. 1.9.4 is the latest and what I am testing with so use that. It may be possible to reduce it.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1638795 13f79535-47bb-0310-9956-ffa450edef68