ci(travis): update osx xcode image (#94)

* ci(travis): update osx xcode image
* ci: use travis's latest supported osx_image xcode11.6
diff --git a/.travis.yml b/.travis.yml
index f64bf39..e813463 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -23,7 +23,7 @@
 
 _ios: &_ios
   os: osx
-  osx_image: xcode10.3
+  osx_image: xcode11.6
 
 _android: &_android
   language: android