Simplify Travis CI macOS homebrew package install

[PeterDaveHello]

Use Travis CI Homebrew addon to install packages, instead of manually install them via shell commands.

According to the latest Travis CI "The macOS Build Environment" and "Installing Dependencies" documentation:

The Travis Homebrew addon is the simplest, fastest and most reliable way to install dependencies.

By default, the Homebrew addon will not run brew update before installing packages. brew update can take a long time and slow down your builds. If you need more up-to-date versions of packages than the snapshot on the build VM has, you can add update: true to the addon configuration:

As Travis CI looks very confident about it, the original brew update corresponding update: true in the addon is not enabled, unless some issues were found without the update, hopefully can speed up the tests.

Reference:

• https://docs.travis-ci.com/user/reference/osx/#homebrew
• https://docs.travis-ci.com/user/installing-dependencies/#installing-packages-on-macos