travis.yml: enable macOS builds again.

.travis.yml

@@ -1,11 +1,23 @@
 language: ruby
-rvm: 2.0.0
-os: linux
+
+matrix:
+  include:
+    - os: osx
+      osx_image: xcode8.0
+      rvm: system
+    - os: linux
+      rvm: 2.0.0
 
 before_install:
   - export HOMEBREW_DEVELOPER=1
-  - export PATH="bin:$PATH"
-  - umask 022
+  - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then
+      HOMEBREW_REPOSITORY="$(brew --repo)";
+      sudo rm -rf "$HOMEBREW_REPOSITORY";
+      ln -s "$PWD" "$HOMEBREW_REPOSITORY";
+    else
+      export PATH="$PWD/bin:$PATH";
+      umask 022;
+    fi
 
 script:
   - brew test-bot