restore "red X" build failures in Travis

...and other travis cleanup.

A while ago I moved the Travis build into scripts under `./scripts/travis`, and in doing so I caused build failures (e.g. compilation errors, lint errors) to show up as "gray" rather than "red".  I still don't really know why this is, as according to the Travis documentation, any non-zero exit codes returned by a command under `script:` should result in the red build failures.  It's also unclear to me why I need the `|| return 1` after `travis_build_and_test.sh` -- it seems redundant -- but without it I see the gray Travis failures, so...