macOS Travis Homebrew cache has expired, failing some builds

The Homebrew cache in the Travis macOS image has expired (or desynced), failing our chutney and stem builds:

Tapped 0 formulae (172 files, 244.5KB)
Error: Your Homebrew is outdated. Please run `brew update`.
Error: Kernel.exit

https://travis-ci.org/torproject/stem/jobs/547493246#L87 https://travis-ci.org/torproject/chutney/jobs/548092876#L95

Our tor builds don't fail, but they aren't running shellcheck on macOS any more. That's probably ok.

Tapped 0 formulae (172 files, 244.5KB)
Error: Your Homebrew is outdated. Please run `brew update`.
Error: Kernel.exit

https://travis-ci.org/torproject/tor/jobs/547856188#L104

There should be a shellcheck here:

sfcgal 1.3.5
sqlite 3.24.0

https://travis-ci.org/torproject/tor/jobs/547856188#L182

There should be a result here:

if command -v shellcheck; then \
                find "." -name "*.sh" -not -path "./src/ext/*" -exec shellcheck {} +; \
	        if [ -d "./scripts/test" ]; then \
                        shellcheck ./scripts/test/cov-diff ./scripts/test/coverage; \
                fi; \
                if [ -e "./contrib/dirauth-tools/nagios-check-tor-authority-cert" ]; then \
                        shellcheck "./contrib/dirauth-tools/nagios-check-tor-authority-cert"; \
                fi; \
                if [ -e "./contrib/client-tools/torify" ]; then \
                        shellcheck "./contrib/client-tools/torify"; \
                fi; \
                if [ -d "./scripts/git" ]; then \
                        shellcheck ./scripts/git/*.git-hook; \
                fi; \
	fi

https://travis-ci.org/torproject/tor/jobs/547856188#L3149

We can fix this in a few different ways:

1. make shellcheck optional in the chutney and stem Travis CI

• it will run on Linux, but not macOS. That's ok.

2. add the update: true key to our Travis macOS builds

• https://docs.travis-ci.com/user/installing-dependencies/#installing-packages-on-macos
• a full update makes macOS builds slower, so we should open a ticket to eventually remove it

3. allow_failure on the chutney and stem macOS Travis jobs

• https://docs.travis-ci.com/user/build-matrix/#rows-that-are-allowed-to-fail