Build.sh: avoid Git's 'Already up-to-date' cluttering version numbers

When calling maven_helper to obtain a version number, the 'Already
up-to-date' of the git pull in scijava-common is everything except
helpful.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Build.sh: if scijava-common could not be updated, fail

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Build.sh: really redirect 'Already up-to-date' message

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Build.sh: avoid unnecessary call to maven_helper

It is already called to determine the appropriate versions.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Build.sh: make sure that just-downloaded files are considered up-to-date

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
download-launchers: fix the version of the ij-launcher.jar

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