Execute tests on various ZSH versions

.travis.yml

@@ -1,23 +1,78 @@
-sudo: true
-dist: trusty
 language: sh
+
+os:
+  - linux
+  - osx
+
 addons:
   apt:
     packages:
       - build-essential
+      - git
+      - mercurial
+      - subversion
+      - jq
+      - node
+      - golang
+      - ruby
+      - python
+      - python-virtualenv
+
+before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update            ; fi
+
+env:
+  global:
+    - ZSH_DIST=$HOME/.zshdist
+  matrix:
+    # Use _ZSH_VERSION since if ZSH_VERSION is present, travis cacher thinks it
+    # is running in zsh and tries to use zsh specific functions.
+    - _ZSH_VERSION=5.5.1
+    - _ZSH_VERSION=5.5
+    - _ZSH_VERSION=5.4.2
+    - _ZSH_VERSION=5.4.1
+    - _ZSH_VERSION=5.3.1
+    - _ZSH_VERSION=5.3
+    - _ZSH_VERSION=5.2
+    - _ZSH_VERSION=5.1.1
+    - _ZSH_VERSION=5.0.8
+    - _ZSH_VERSION=5.0.2
+
+cache:
+  directories:
+    - $ZSH_DIST
 
 before_script:
+  - >
+    setup_zsh() {
+      dest="$ZSH_DIST/$1"
+      if [[ ! -d $dest/bin ]]; then
+        coreutils_mktemp="mktemp"
+        if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+          coreutils_mktemp="gmktemp"
+        fi
+        tmp="$(${coreutils_mktemp} --directory --tmpdir="${TMPDIR:/tmp}" zshbuild.XXXXXX)"
+        (
+          cd "$tmp" &&
+          curl -L http://downloads.sourceforge.net/zsh/zsh-${1}.tar.gz | tar zx &&
+          cd zsh-$1 &&
+          ./configure --prefix="$dest" &&
+          make &&
+          mkdir -p "$dest" &&
+          make install ||
+          echo "Failed to build zsh-${1}!"
+        )
+      fi
+      export PATH="$dest/bin:$PATH"
+    }
+  - setup_zsh $_ZSH_VERSION
   # Show the git version being used to test.
   - "git --version"
+  # Show the mercurial version being used to test.
+  - "hg --version"
   # Show the zsh version being used to test.
   - "zsh --version"
 
-install:
-  - "sudo apt-get update -qq"
-  - "sudo apt-get install -y zsh"
-  - "sudo chsh -s $(which zsh)"
-  - "sudo apt-get install -y git mercurial subversion jq node golang ruby python python-virtualenv"
-
 script:
   - test/powerlevel9k.spec
   - test/functions/utilities.spec