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.
Merge pull request #1817 from gildegoma/travis-ci_use_build_dir_variable

Travis CI: Use new $TRAVIS_BUILD_DIR environment variable