travis: downgraded MacOS release build to XCode11.1 to make it fit into the time limit

Author: TheMostDiligent

.travis.yml

@@ -43,7 +43,7 @@ matrix:
     env:
       - CONFIG=Release
   - os: osx
-    osx_image: xcode12.2
+    osx_image: xcode11.1
     compiler: clang
     env: 
       - CONFIG=Release