travis-script.sh: respect -j in install_from_tarball()
travis-script.sh: respect -j in install_from_tarball()

I didn't modify this when I added -j support, but @23Skidoo agreed it
should be changed, so we do it now.

kolmodin/cabal,mydaum/cabal,thomie/cabal,themoritz/cabal,sopvop/cabal,themoritz/cabal,themoritz/cabal,kolmodin/cabal,headprogrammingczar/cabal,mydaum/cabal,sopvop/cabal,thomie/cabal,headprogrammingczar/cabal,sopvop/cabal