Removed old macOS build, added latest

Homebrew complains about our attempt to build with the old version and
gets stuck during the installation of some components.

- removed Xcode 10.2.1 on macOS 10.14
- added Xcode 13.1 on macOS 11.6

.travis.yml

@@ -12,13 +12,13 @@ addons:
 
 matrix:
   include:
-    # Xcode 10.2.1 on macOS 10.14
+    # Xcode 12.0.1 on macOS 10.15.7
     - os: osx
-      osx_image: xcode10.2
+      osx_image: xcode12
 
-    # Xcode 12.0 on macOS 10.15.5
+    # Xcode 13.1 on macOS 11.6
     - os: osx
-      osx_image: xcode12
+      osx_image: xcode13.1
 
     # Ubuntu 20.04 LTS, gcc, x64
     - os: linux