Update Travis Xcode version

Charlotte Tortorella · Charlotte Tortorella · commit f908b3be8fdf · 2019-04-17T14:01:31.000+10:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,5 +1,5 @@
 language: objective-c
-osx_image: xcode9.3
+osx_image: xcode10.2
 
 branches:
   except:
