Pin to Xcode 8.3 so that OSXFUSE works again

Travis CI has updated its macOS workers to Xcode 9.2, which requires
macOS 10.13.  This new macOS version requires explicit approval to
enable a kernel extension and we don't know yet of a way to grant
such permission programmatically.  Therefore, it's easier to stick
to an older version for now.

See travis-ci/travis-ci#10017 for details.

.travis.yml

@@ -18,6 +18,14 @@ os:
   - linux
   - osx
 
+# Pin to macOS 10.12 (indirectly by using the Xcode 8.3 image).  We must do
+# this to get the OSXFUSE kernel extension to work because there currently
+# is no know way to programmatically grant permissions to load a kernel
+# extension in macOS 10.13.
+#
+# See https://github.com/travis-ci/travis-ci/issues/10017 for details.
+osx_image: xcode8.3
+
 language:
   - go  # For ourselves.
   - java  # For Bazel.