.travis.yml: enable macOS builds too.

.travis.yml

 # Cf. http://docs.travis-ci.com/user/getting-started/
 # Cf. http://docs.travis-ci.com/user/languages/go/
 
-language: go
-
-go: 1.8.1
-
-# Use the virtualized Trusty beta Travis is running in order to get support for
-# installing fuse.
-#
-# Cf. Personal communication from support@travis-ci.com.
-dist: trusty
+matrix:
+  include:
+    - os: linux
+      language: go
+      go: 1.8.1
+      # Use the virtualized Trusty beta Travis is running in order to get
+      # support for installing fuse.
+      #
+      # Cf. Personal communication from support@travis-ci.com.
+      dist: trusty
+      sudo: required
+    - os: osx
+      language: go
+      go: 1.8.1
 
 # Install fuse before installing our code.
 before_install:
-  - sudo apt-get install -qq fuse
+  # For linux: install fuse.
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
+      sudo apt-get install -qq fuse;
+    fi
+
+  # For macOS: update homebrew and then install osxfuse.
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew cask install osxfuse; fi