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

Change-Id: I14c9635b56dac2c7988734d72adf8c41c5c5540e
diff --git a/.travis.yml b/.travis.yml
index 8990ac7..5b2de71 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