Fix_Updater.js: try even harder to get out of a fix

Under certain circumstances, the PluginClassLoader's URL array
becomes empty, even if the PluginClassLoader itself has no
problem finding classes. But Javascript will not be able to
execute the otherwise innocuous 'importPackage(Packages.ij);'
statement.

So let's try extra, extra hard to find the ij.IJ class.

This is related to the bug reported by Claudio Collinet in

	http://fiji.sc/bugzilla/show_bug.cgi?id=763

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