Try turning on OS X testing on travis.

.travis.yml

@@ -108,12 +108,12 @@ matrix:
     ###################
     # # Disabled because travis's OS X machines take hours (or days) to begin
     # running.  Or maybe they are just broken entirely.  Who knows.
-    #- language: cpp
-    #  os: osx
-    #  osx_image: xcode9.2
-    #  env:
-    #    - VARIANT=test
-    #  script:
-    #    - dlib/travis/build-and-test.sh
+    - language: cpp
+      os: osx
+      osx_image: xcode9.2
+      env:
+        - VARIANT=test
+      script:
+        - dlib/travis/build-and-test.sh