travis: update Homebrew explicitly (#1081)
travis: update Homebrew explicitly (#1081)

Workaround for this issue:
https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296
kenhys/groonga,naoa/groonga,groonga/groonga,groonga/groonga,naoa/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,groonga/groonga,groonga/groonga,groonga/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,naoa/groonga,naoa/groonga,komainu8/groonga,kenhys/groonga