Add fork=true to insight's build

Under Windows, the error:

 `com.sun.tools.javac.Main is not on the classpath`

can be printed depending on the Java installation.

John Webber (NBI) suggested adding fork=true.
See: http://lists.openmicroscopy.org.uk/pipermail/ome-users/2013-May/003699.html
Merge pull request #1156 from joshmoore/insight-javac-fork

Add fork=true to insight's build