Updater bootstrap.js: set the imagej.dir property in addition to ij.dir

In the future, the ImageJ launcher will set imagej.dir (and only set
ij.dir and fiji.dir for backwards-compatibility). We should rely on
the imagej.dir property instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater bootstrap.js: when imagej.dir is not set, look at ij.dir first

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