issue #29 - Make Travis test on macOS.

And mercy be with us all.

 - Use `apt` and `homebrew` addons for simplicity and build speed (?).
 - Make build matrix explicit: can't handle installing different
   versions of fish otherwise.

.travis.yml

@@ -1,16 +1,40 @@
-env:
-    - FISH=release-2
-    - FISH=release-3
-
 matrix:
+  include:
+    - os: linux
+      env: FISH=2
+      addons:
+        apt:
+          sources:
+            - sourceline: "ppa:fish-shell/release-2"
+          packages:
+            - fish
+    - os: linux
+      env: FISH=3
+      addons:
+        apt:
+          sources:
+            - sourceline: "ppa:fish-shell/release-3"
+          packages:
+            - fish
+    - os: osx
+      env: FISH=2
+      addons:
+        homebrew:
+          packages:
+            - 'https://raw.githubusercontent.com/Homebrew/homebrew-core/799fef191cd2beac06930e1d1a8e7f308bd0f4b1/Formula/fish.rb' # 2.7.1
+          update: true # TODO: build should be green without, but isn't
+    - os: osx
+      env: FISH=3
+      addons:
+        homebrew:
+          packages:
+            - fish # --> latest, i.e. >=3.0.2
+          update: true # TODO: build should be green without, but isn't
   allow_failures:
-        - env: FISH=release-3
+    - env: FISH=3
 
-sudo: required
 before_install:
-    - sudo add-apt-repository -y ppa:fish-shell/${FISH}
-    - sudo apt-get update
-    - sudo apt-get -y install fish
+  - fish --version
   - curl -s "https://get.sdkman.io" | bash
   - bash test/prepare_tests.sh
 
@@ -25,4 +49,3 @@ script:
   - fish test/wrapper.fish
   - fish test/reinitialize.fish
   - bash -c "sdk install crash 1.3.0; sdk uninstall crash 1.3.0"; fish test/10_zombies_new.fish
-