ci(travis): update osx xcode image (apache#768)

* ci(travis): update osx xcode image
* ci: use travis's latest supported osx_image xcode11.6

Author: erisu

.travis.yml

@@ -23,7 +23,7 @@ node_js: 12
 
 _ios: &_ios
   os: osx
-  osx_image: xcode10.3
+  osx_image: xcode11.6
 
 _android: &_android
   language: android