Add xcode12.3 test on macOS Big Sur

Author: bobrik

.travis.yml

@@ -31,9 +31,13 @@ matrix:
     osx_image: xcode10.3
     env: CMAKE_BUILD_TYPE=Release
   - os: osx
-    name: "CMake Release Test on OSX Catalina"
+    name: "CMake Release Test on macOS Catalina"
     osx_image: xcode12.2
     env: CMAKE_BUILD_TYPE=Release
+  - os: osx
+    name: "CMake Release Test on macOS Big Sur"
+    osx_image: xcode12.3
+    env: CMAKE_BUILD_TYPE=Release
   - os: osx
     osx_image: xcode10.3
     name: "Makefile Test on Mac OS X"