Don't update homebrew on macOS travis builds.
The macOS system image already provides a version of xz through homebrew, so we don't need to explicitly install it through `brew bundle`. Skipping the `brew update` step reduces the runtime of test jobs from 6 minutes to 1, so we get test feedback more promptly. The xz package is really only needed for 'make dist' targets, so listing it in a Brewfile doesn't help users get started building the library. Better not to clutter the top-level directory further.
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