Travis: add a build on macOS

Just do the default build with Clang and run the unit tests. The
objective is to have one build on a Unix-like platform other than
Linux.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.travis.yml

@@ -32,6 +32,12 @@ jobs:
         - tests/scripts/test-ref-configs.pl
         - tests/scripts/all.sh -k 'test_depends_*' 'build_key_exchanges'
 
+    - name: macOS
+      os: osx
+      compiler: clang
+      script:
+        - tests/scripts/all.sh -k test_default_out_of_box
+
 after_failure:
 - tests/scripts/travis-log-failure.sh