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

Change-Id: Ied88268107c94523ea651fae101274083d12f8fd
diff --git a/.travis.yml b/.travis.yml
index ab3e066..fd734d6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -44,6 +44,12 @@
     - os: osx
       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:
     - env: COMPILER=g++-5