Only start Xvfb on Linux.

Author: octalmage

.travis.yml

@@ -42,8 +42,8 @@ before_install:
 
 before_script:
   # Start xvfb
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
+  - if [[ $TRAVIS_OS_NAME == "linux" ]]; then export DISPLAY=:99.0; fi
+  - if [[ $TRAVIS_OS_NAME == "linux" ]]; then sh -e /etc/init.d/xvfb start; fi
 
 install:
   - npm install