Bump to pom-imagej 2.15 to get a newer version of scripting-java

This is needed to support developing .java plugins in the script editor,
as scripting-java and MiniMaven got bug fixes for that use case in the
meantime. The updated versions are already referenced in a new
pom-scijava, but pom-imagej was not yet updated accordingly (waiting for
the very topic branches in imagej-ui-swing and imagej-legacy for which
the bug fixes were developed).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>