Turn on macOS builds for Travis (GH-1846)

Initially the macOS builds are allowed to fail until such time that they can be determined to be stable and not add an unacceptable amount of time to the overall Travis-passing process.

.travis.yml

@@ -14,22 +14,25 @@ branches:
 matrix:
   fast_finish: true
   allow_failures:
-    - env:
-       - TESTING=coverage
+    - env: OPTIONAL=true
   include:
     - os: linux
       language: c
       compiler: clang
       # gcc also works, but to keep the # of concurrent builds down, we use one C
-      # compiler here and the other to run the coverage build.
-      env:
-        - TESTING=cpython
+      # compiler here and the other to run the coverage build. Clang is preferred
+      # in this instance for its better error messages.
+      env: TESTING=cpython
+    - os: osx
+      language: c
+      compiler: clang
+      # Testing under macOS is optional until testing stability has been demonstrated.
+      env: OPTIONAL=true
     - os: linux
       language: python
       # Build the docs against a stable version of Python so code bugs don't hold up doc-related PRs.
       python: 3.6
-      env:
-        - TESTING=docs
+      env: TESTING=docs
       before_script:
         - cd Doc
         # Sphinx is pinned so that new versions that introduce new warnings won't suddenly cause build failures.
@@ -40,8 +43,7 @@ matrix:
     - os: linux
       language: c
       compiler: gcc
-      env:
-        - TESTING=coverage
+      env: OPTIONAL=true
       before_script:
         - |
             if ! git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(\.(rst|yml)$)|(^Doc)|(^Misc)/'