Improve build script
Improve build script

This small change makes it more comfortable to run the build script with
additional Maven options. To do so, simply call the script like

    $ ADDITIONAL_MVN_OPTS="-DskipTests -U" ./build-all.sh

scala-ide/scala-ide,scala-ide/scala-ide,mlangc/scala-ide,wpopielarski/scala-ide,scala-ide/scala-ide,sschaef/scala-ide,scala-ide/scala-ide,mlangc/scala-ide,sschaef/scala-ide,wpopielarski/scala-ide,wpopielarski/scala-ide,sschaef/scala-ide,mlangc/scala-ide,sschaef/scala-ide,wpopielarski/scala-ide,mlangc/scala-ide