Restore travis config, using TRAVIS_BUILD_DIR directly

"The fix for this has been rolled out, let us know if you're still seeing any issues!" </snip>

Revert "Derive $NUPIC from $PWD and $TRAVIS_REPO_SLUG as https://github.com/travis-ci/travis-build/pull/182 may not be deployed yet"

This reverts commit 65ba48645eb29f64062679646a8b8335c4af1026.