Use the Homebrew addon on Travis

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`

+ $ if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install libusb ; fi
+ 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.

.travis.yml

@@ -13,10 +13,10 @@ addons:
         update: true
         packages:
         - libusb-1.0-0-dev
-
-before_install:
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update         ; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install libusb ; fi
+    homebrew:
+        packages:
+        - libusb
+        update: true
 
 script:
     - make