core: enable fast_finishing (Travis CI)

fash_finish makes it so that if one job fails, the build is marked as
failed without having to wait for the other jobs to finish. If there are
still unfinished jobs at the time of failure, they continue running
after the build has been marked as failed.
tests: make build.sh show executed commands & ...

...try to speed up make by using multiple jobs if there is more than one
CPU available

Travis CI should now show the commands that build.sh executes in the
log. I mean commands, not only their output.
core: add empty line in .travis.yml
core: remove "set -x" in build.sh (the executed commands are already displayed)