Replace Travis CI with GitHub Actions

* Use GH Actions Matrices for multi-OS & multi-shell testing
* Drop helper script for installing zsh on Linux
    - Should be available by default on ubuntu-latest

test/support/travisci_deps.sh

@@ -1,32 +0,0 @@
-#!/usr/bin/env bash
-# Installs dependencies for travis-ci environments.
-
-# Install dependencies, which looks to be just bash & zsh.
-#
-# Darwin has zsh preinstalled already, so only need to install on Ubuntu.
-#
-# Note: $TRAVIS_OS_NAME will only be set on text boxes with multi-os enabled,
-# so use negation test so it will fail gracefully on normal Travis linux setup.
-if [[ "$TRAVIS_OS_NAME" != "osx" ]]; then
-
-  # okay, so we know we're probably on a linux box (or at least not an osx box)
-  # at this point. do we need to install zsh? let's say the default case is no:
-  needs_zsh=false
-
-  # check if zsh is listed in the TEST_SHELLS environment variable, set by
-  # our travis-ci build matrix.
-  if [[ $TEST_SHELLS =~ zsh ]]; then needs_zsh=true; fi
-
-  # if there is NO $TEST_SHELLS env variable persent (which should never happen,
-  # but maybe someone has been monkeying with the .travis.yml), run_tests.sh is
-  # going to fall back onto the default of testing everything, so we need zsh.
-  if [[ -z "$TEST_SHELLS" ]]; then needs_zsh=true; fi
-
-  # finally, we install zsh if needed!
-  if $needs_zsh; then
-    sudo apt-get update
-    sudo apt-get install zsh
-  else
-    echo "No deps required."
-  fi
-fi