Use build script from payload if one is provided
Use build script from payload if one is provided

This allows us to move the compilation of build scripts to hub, which
again means that updating travis-build doesn't require us to update all
the workers, allowing for quicker and more easy deployments of
travis-build.

gavioto/travis-worker,final-ci/travis-worker,final-ci/travis-worker,gavioto/travis-worker,ireznice/travis-worker,ireznice/travis-worker,travis-ci/travis-worker,travis-ci/travis-worker