travis needs the homebrew update option set true
travis needs the homebrew update option set true

Else we get the following stack:
```
brew
Installing Homebrew Packages
6.51s$ rvm $brew_ruby do brew bundle --verbose --global
/usr/local/bin/brew tap homebrew/bundle
==> Tapping homebrew/bundle
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-bundle'...
remote: Enumerating objects: 101, done.
remote: Counting objects: 100% (101/101), done.
remote: Compressing objects: 100% (95/95), done.
remote: Total 101 (delta 4), reused 28 (delta 1), pack-reused 0
Receiving objects: 100% (101/101), 51.37 KiB | 2.23 MiB/s, done.
Resolving deltas: 100% (4/4), done.
Tapped (109 files, 268.1KB).
Error: Unknown command: bundle
before_install.1
0.01s$ echo $PATH
0.05s$ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
    eval "$(/usr/libexec/path_helper)"
    sudo tlmgr update --self
  elif [ "$TRAVIS_OS_NAME" = "linux" ]; then
    tlmgr init-usertree
    echo MSG FROM JO:
    echo on linux we cannot update apt installed texlive
    echo next steps to fix see commit msg: https://github.com/tompollard/phd_thesis_markdown/pull/100/commits/85fd9e7ac413a34066b79f1e49b3e1d3efdeac00
    echo c.f. https://tex.stackexchange.com/questions/551383/cant-run-tex-lives-tlmgr-in-a-github-action
    echo and https://tex.stackexchange.com/questions/1092/how-to-install-vanilla-texlive-on-debian-or-ubuntu

  fi

sudo: tlmgr: command not found
```

tompollard/phd_thesis_markdown,tompollard/phd_thesis_markdown,tompollard/phd_thesis_markdown