This PR makes it so we _always_ build manylinux wheels, but only upload
them for git tags.

The diff is messy, for which I apologize.  I had to reformat the YAML to
understand it.  Highlights:

- Moved regular Linux Python builds outside build matrix, for
  conciseness (unfortunately this means 3.6 builds are now done twice:
  once without WITH_COVERAGE and once with)

- Move twine upload from .manylinux.sh into .travis.yml's after_success,
  next to the twine upload used for Mac OS builds

- This means Travis can no longer do an 'exit 0' in before_install, so I
  had to override the install and script sections too

- Move 'docker pull' into the install section so it's not so very empty

- Unify the two calls to twine upload to provide the twine password via
  environment variables instead of command-line or configuration files

- Put related commands inside a single multiline if statement instead of
  repeating the conditions