Fix jsgui build on Windows take 2

Replaces the custom bash script with an invocation of the gzip ant task.

Could somebody that uses Windows test building this? I expect the build to succeed and four gzipped files to be present in target:
> sam@v:~/code/brooklyn.io/brooklyn/usage/jsgui (jsgui-windows *)
$ find target -name *.gz
target/brooklyn-jsgui-0.7.0-SNAPSHOT/assets/css/brooklyn.css.gz
target/brooklyn-jsgui-0.7.0-SNAPSHOT/assets/css/styles.min.css.gz
target/brooklyn-jsgui-0.7.0-SNAPSHOT/assets/js/gui.min.js.gz
target/brooklyn-jsgui-0.7.0-SNAPSHOT/assets/js/libs/require.js.gz