MiniMaven: accelerate repeated upToDate() checks

... by remembering the previous result.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven: be less zealous in buildAndInstall()

When the artifacts are already up-to-date we do not need to rebuild
them...

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