Travis: Update build enviourment to xcode11
diff --git a/.travis.yml b/.travis.yml
index 3003a37..32a4db1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: objective-c 
 
 # macOS and Xcode Version
-osx_image: xcode10
+osx_image: xcode11
 
 # Build dependencies
 # Travis has trouble with python3, which SPIRV-Tools requires,