Remove travis build now RTD is in place

[thclark]

Now that es-flow is on public release, remove the old travis build process.

travis.yml file to build the docs was:

language: cpp
branches:
  only:
  - "/^release-.*$/"
git:
  depth: 10
matrix:
  include:
  - name: 'JOB: Build docs for release-<tag> branches and deploy to tagged release
      (uses Python 2.7)'
    if: "(branch =~ /^release-.*$/) AND (type != pull_request)"
    os: osx
    env: PYTHON=2.7 HOMEBREW_NO_AUTO_UPDATE=1
    stage: GitHub Release
    install:
    - "(brew install pyenv || brew upgrade pyenv) && (brew install pyenv-virtualenv
      || brew upgrade pyenv-virtualenv) && (brew install doxygen || brew upgrade doxygen)"
    - pyenv install --skip-existing $PYTHON
    - pyenv versions
    - pyenv virtualenv $PYTHON venv && eval "$(pyenv init -)" && pyenv activate venv
    - pip install -r requirements.txt
    script:
    - export TRAVIS_TAG=${TRAVIS_BRANCH#release-} && echo "Deploying to Github release with TRAVIS_TAG=$TRAVIS_TAG"
    - export ES_FLOW_ROOT=`pwd` && echo "Set ES_FLOW_ROOT to $ES_FLOW_ROOT"
    - cd $ES_FLOW_ROOT/docs/source
    - python $ES_FLOW_ROOT/scripts/make_docs.py $ES_FLOW_ROOT/docs/source $ES_FLOW_ROOT/docs $TRAVIS_TAG
    - cd ../..
    before_deploy:
    - rm -r docs/html/_sources
    - mkdir -p build/docs
    - mkdir -p build/include
    - mkdir -p build/lib
    - mkdir -p build/bin
    - mv docs/html build/docs
    - ls
    deploy:
      provider: s3
      skip_cleanup: true
      access_key_id: AKIAIMZGKM7HOVQHYUNA
      secret_access_key:
        secure: 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
      bucket: octue-builds
      region: eu-west-1
      local_dir: build
      upload_dir: es-flow/$TRAVIS_TAG
      acl: private
      on:
        repo: octue/es-flow
        all_branches: true