Add CI for macOS.

.travis.yml

@@ -1,3 +1,6 @@
+os:
+  - linux
+  - osx
 language: node_js
 node_js:
   - 8
@@ -5,8 +8,7 @@ sudo: required
 addons:
     chrome: stable
 before_script:
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
+  - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then export DISPLAY=:99.0; sh -e /etc/init.d/xvfb start; fi
 install:
   - npm set progress=false
   - npm install