Also run tests on macOS.

However, allow them to fail (or hang/timeout, if there aren't enough
macOS compute resources), so that our progress isn't blocked, since
Linux VMs are plentiful.

.travis.yml

@@ -12,10 +12,23 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+dist: bionic
+sudo: false
+
 language: generic
 
+os:
+  - linux
+  - osx
+
 branches:
   only:
     - master
 
+matrix:
+  allow_failures:
+    - os: osx
+
+  fast_finish: true
+
 script: make test