Use the declarative support for installing packages from
homebrew on the travis-ci.org continuous integration service
macOS images, rather than conditional shell.

This also removes the `brew update` step. There's support
for that as well with the `update: true` key-value pair,
but since travis doesn't update their images often, the
base set of homebrew packages is often very out of date,
and the corresponding update step can take a very long time.