update xcode version to 9.4
diff --git a/.travis.yml b/.travis.yml
index 2338541..66d7946 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
 sudo: required
 dist: trusty
 services: docker
-osx_image: xcode8
+osx_image: xcode9.4
 
 env:
   global: