release-version: name the version parameter before using it

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
release-version: use some sensible defaults

As per ImageJ's RELEASES.md, but really, the defaults are open to any
project wanting to be part of the SciJava consortium.

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