Get rid of `publish-travis.sh`

[kailuowang]

The bash script is getting unmanageable.
Instead we can define travis jobs explicitly in a clearer jobs spec, here is a simplified example.

scala: 
    - 2.11.12
    - 2.12.6
    - 2.13.0-M4
jobs:
  include:
    - env:
        - NAME="Validate JVM"
      script:
        - sbt ++$TRAVIS_SCALA_VERSION validateJVM

    - if: env(TRAVIS_SCALA_VERSION) =~ /^2.12/ 
      env:
        - NAME="Validate docs"
      script:
        - sbt docs/tut

    - if: (branch = master) AND (type = pull_request)
      env:
        - NAME="Publish SNAPSHOT"
      script:
        - sbt ++$TRAVIS_SCALA_VERSION publish gitSnapshots publish

[djspiewak]

FWIW, there is no filesystem caching between job stages, and even if you emulate it by hand (e.g. by using S3 to pass files between stages), you still run into the fact that Zinc is irrationally picky about file state and will often refuse to incrementally compile.

[ceedubs]

In case it saves anyone else some time, I went down a bit of a rabbit hole trying to use Travis CI's new "stages" feature, and ran into trouble. I don't entirely understand it, but it seems that the "stages" are part of the build matrix, as is the scala version, and there's some sort of interaction where it tries to create a default stage that just runs test no matter what you do. Or at least that was my experience. Someone who knows more about Travis may be able to do better.

[ceedubs]

I'm working on this. I'll submit a PR after #2732 is merged.

[ceedubs]

On that note: does anyone know why our publish command is publish gitSnapshots publish? Why does publish show up twice?

Edit: I'm guessing it's probably so that we always publish an x.y-SNAPSHOT with the latest changes and separately a x.y-<commit-hash>-SNAPSHOT.