Comment out osx xcode 9.4

.travis.yml

@@ -66,8 +66,8 @@ matrix:
         - libhiredis-dev
   - os: osx
     osx_image: xcode10.3
-  - os: osx
-    osx_image: xcode9.4
+#  - os: osx
+#    osx_image: xcode9.4
 
 notifications:
   slack: