Travis: Add kitware repository to source list

The default distribution of CMake that comes with focal is 3.16.3. Add
kitware's repository (https://apt.kitware.com/) to the source list so
the latest copy of CMake is installed during CI.

.travis.yml

@@ -35,6 +35,9 @@ before_install:
 
 addons:
   apt:
+    sources:
+      - sourceline: 'deb https://apt.kitware.com/ubuntu/ focal main'
+        key_url: 'https://apt.kitware.com/keys/kitware-archive-latest.asc'
     packages:
     - cmake
     - ninja-build