ci: update xcode image to 11.5 (#102)

diff --git a/.travis.yml b/.travis.yml
index 97faef0..b0d2ab8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,5 @@
 language: objective-c
-osx_image: xcode9.4
+osx_image: xcode11.5
 sudo: false
 
 env: