Upgrade CI to modern macOS

[jmmv]

Our Travis CI builds for macOS currently rely on the image that provides El Capitan because there is no known mechanism to load the FUSE kext without interacting with the UI (https://github.com/travis-ci/travis-ci/issues/10017).

This is problematic because we cannot use modern APIs in our code (things like futimensat), which are necessary to implement and/or test certain semantics.