ImgLib2-ij is versioned independently of ImgLib2 now

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Temporarily switch to explicit ImgLib2 version

ImgLib2 beta 26 introduces API breaking changes; therefore we need to
switch to a newer version explicitly, without relying on pom-scijava
to list the proper version.

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