Avoid deprecated Data::getSignature()

Change-Id: Id644d5bd4414ed97d70ed108637516199d806be9
diff --git a/.travis.yml b/.travis.yml
index 7c63efd..9eb8ffe 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -38,6 +38,12 @@
     - os: osx
       osx_image: xcode11.3
       env: # default compiler
+    - os: osx
+      osx_image: xcode11.6
+      env: # default compiler
+    - os: osx
+      osx_image: xcode12
+      env: # default compiler
 
   allow_failures:
     - env: COMPILER=clang++-11