install latest version of chrome for osx in travis

.travis.yml

@@ -17,10 +17,12 @@ services:
 before_install:
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then docker build -t zixia/wechaty .; fi
 
+install:
+  - if [ "$TRAVIS_OS_NAME" == 'osx' ]; then brew install brew-cask && brew cask install google-chrome; fi
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then export DISPLAY=':99.0' && Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &; fi
+
 script:
   - echo $TRAVIS_OS_NAME
-  - export DISPLAY=':99.0'
-  - Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
   - npm test
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then docker run zixia/wechaty test; fi