ci: add Xcode 11.6 and 12 to Travis CI build matrix

Change-Id: I37add5d807b36dbd8dc8e9f3248488ca5d8a46e5
diff --git a/.travis.yml b/.travis.yml
index 7927f10..e8c6344 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -42,7 +42,13 @@
       osx_image: xcode11.3
       env: # default compiler
     - os: osx
-      osx_image: xcode11.5
+      osx_image: xcode11.4
+      env: # default compiler
+    - os: osx
+      osx_image: xcode11.6
+      env: # default compiler
+    - os: osx
+      osx_image: xcode12
       env: # default compiler
 
   allow_failures: