.travis.yml: pin to macOS 10.12

.travis.yml

@@ -13,6 +13,13 @@ matrix:
       dist: trusty
       sudo: required
     - os: 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
       go: 1.13