Neaten the dist/travis.sh presentation

[spl]

• Make the Travis-CI log easier to follow by reducing the noise:
  • put more things behind folds
  • put folds inside if-then blocks to avoid printing '+ false', etc.
  • make fold titles simpler and more consistent
  • avoid printing the values already found in the printed env vars
• Document the travis_fold usage

[pkgw]

I had chosen to put the fold markers outside of the if statements because it helped me understand why certain steps were running, or not running, when I straightened out the Travis infrastructure, but at this point I think the script is a lot better-structured so those markers do seem less necessary.

[spl]

@pkgw Sure, I understand. You can always add more information to the fold titles if it would help you. I would just recommend making the text more meaningful than true or false so that one doesn't have to look at the script source to figure out what variables are being evaluated to get those values.