Travis CI: Use new $TRAVIS_BUILD_DIR env-variable
Travis CI: Use new $TRAVIS_BUILD_DIR env-variable

Before this fix: environment variables defined `.travis.yml` were still
dependant of internal configuration of the travis worker virtual
machine. See also previous pull request #1728.

With this fix, `.travis.yml` is even more generic and portable.

wangshijin/cappuccino,koamac/Cappuccino,mrcarlberg/cappuccino,kidaa/cappuccino,Dogild/cappuccino,primalmotion/cappuccino,kidaa/cappuccino,tancred/cappuccino,wangshijin/cappuccino,primalmotion/cappuccino,cacaodev/cappuccino,koamac/Cappuccino,cacaodev/cappuccino,tancred/cappuccino,zittix/cappuccino,deltaprojects/cappuccino,cacaodev/cappuccino,Dogild/cappuccino,rgv151/cappuccino,tancred/cappuccino,daboe01/cappuccino,Dogild/cappuccino,tancred/cappuccino,kidaa/cappuccino,daboe01/cappuccino,primalmotion/cappuccino,Rosch/cappuccino,deltaprojects/cappuccino,Rosch/cappuccino,FrankReh/cappuccino,Dogild/cappuccino,cappuccino/cappuccino,daboe01/cappuccino,mrcarlberg/cappuccino,krodelin/cappuccino,kidaa/cappuccino,rgv151/cappuccino,FrankReh/cappuccino,daboe01/cappuccino,rgv151/cappuccino,mrcarlberg/cappuccino,wangshijin/cappuccino,mrcarlberg/cappuccino,primalmotion/cappuccino,Rosch/cappuccino,kidaa/cappuccino,FrankReh/cappuccino,tancred/cappuccino,Rosch/cappuccino,enquora/cappuccino,cappuccino/cappuccino,FrankReh/cappuccino,deltaprojects/cappuccino,krodelin/cappuccino,AlexanderMazaletskiy/cappuccino,enquora/cappuccino,Rosch/cappuccino,primalmotion/cappuccino,AlexanderMazaletskiy/cappuccino,cappuccino/cappuccino,AlexanderMazaletskiy/cappuccino,deltaprojects/cappuccino,primalmotion/cappuccino,mrcarlberg/cappuccino,rgv151/cappuccino,koamac/Cappuccino,kidaa/cappuccino,cacaodev/cappuccino,deltaprojects/cappuccino,Dogild/cappuccino,krodelin/cappuccino,cappuccino/cappuccino,daboe01/cappuccino,cacaodev/cappuccino,Rosch/cappuccino,cappuccino/cappuccino,rgv151/cappuccino,AlexanderMazaletskiy/cappuccino,enquora/cappuccino,tancred/cappuccino,zittix/cappuccino,enquora/cappuccino,FrankReh/cappuccino,AlexanderMazaletskiy/cappuccino,deltaprojects/cappuccino,Dogild/cappuccino,zittix/cappuccino,wangshijin/cappuccino,zittix/cappuccino,mrcarlberg/cappuccino,rgv151/cappuccino,cacaodev/cappuccino,cacaodev/cappuccino,mrcarlberg/cappuccino,zittix/cappuccino,daboe01/cappuccino,zittix/cappuccino,FrankReh/cappuccino,daboe01/cappuccino,Dogild/cappuccino,mrcarlberg/cappuccino,rgv151/cappuccino,AlexanderMazaletskiy/cappuccino,FrankReh/cappuccino,krodelin/cappuccino,enquora/cappuccino,enquora/cappuccino,wangshijin/cappuccino,wangshijin/cappuccino,cappuccino/cappuccino,krodelin/cappuccino,zittix/cappuccino,enquora/cappuccino,krodelin/cappuccino,Rosch/cappuccino,FrankReh/cappuccino,primalmotion/cappuccino,deltaprojects/cappuccino,krodelin/cappuccino,koamac/Cappuccino,koamac/Cappuccino,koamac/Cappuccino,cappuccino/cappuccino