Whoopsie. Bump ui-swing *again*

There is a rather critical bug fix in imagej-ui-swing 0.4.6: when
updating the updater itself, we make a new class loader, but we used
the former class loader as parent class loader by mistake: no new
classes were loaded at all; the old ImageJ updater classes were
loaded.

Unfortunately this means that everybody who tries to update Fiji needs
to cancel the infinite loop now, *quit ImageJ*, restart and try again.

At least it works that way, and the included bug fix will prevent that
infinite loop for the future.

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