MiniMaven: make the forceBuild behavior consistent

We changed buildAndInstall() to build only when there are out-of-date
files, let's do the same with buildJar() and build(), too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven: make 'clean' work in aggregate projects

Calling the 'clean' goal of MiniMaven for aggregate projects clearly
wants to iterate the modules.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>