Use the Homebrew addon on Travis

[jonasmalacofilho]

Proposed changes

With the latest release of libusb on Homebrew, a warning when ensuring it's installed became an error.

  $ if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install libusb ; fi
- Warning: libusb 1.0.22 is already installed and up-to-date
- To reinstall 1.0.22, run `brew reinstall libusb`
+ Error: libusb 1.0.22 is already installed
+ To upgrade to 1.0.23, run `brew upgrade libusb`.

Instead of manually dealing with this, delegate to Travis.

Types of changes

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] Make sure you are requesting to pull a topic/feature/bugfix branch (right side). Don't request your master!
• [x] Make sure you are making a pull request against the testing branch (left side). Also you should start your branch off our testing.
• [ ] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] Check the commit's or even all commits' message styles matches our requested structure.