travis.yml: remove obsolete brew-cask install

.travis.yml

@@ -31,7 +31,6 @@ matrix:
       script:
         - unset -f cd # workaround for https://github.com/travis-ci/travis-ci/issues/8703
         - brew update
-        - brew install caskroom/cask/brew-cask
         - brew cask install osxfuse
         - go run build/ci.go install
         - go run build/ci.go test -coverage $TEST_PACKAGES