Require Ant >= 1.8.0

Reportedly our build fails with Ant 1.7.1. Time to raise the
bar. Let's party like it was 2010!

Tested by temporarily setting the requirement to 1.8.5 and observing:

    % ant init
    Buildfile: /Users/jason/code/scala/build.xml

    desired.jars.uptodate:

    boot:

    init:

    BUILD FAILED
    /Users/jason/code/scala/build.xml:216: Ant version 1.8.5 is required. You are running Apache Ant(TM) version 1.8.4 compiled on May 22 2012