Fixing macOS Travis Build

- cask gcc-arm-embedded package was removed
  Homebrew/homebrew-cask#56802
- Now using armmbed/formulae

.travis.yml

@@ -71,9 +71,9 @@ before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew upgrade python; fi # Homebrew defaults to Python 3 now, but we have to upgrade
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install tree ctags dfu-util binutils gnu-sed; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]] && [[ "$CC" == "gcc" ]]; then brew tap Caskroom/cask; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]] && [[ "$CC" == "gcc" ]]; then brew tap armmbed/formulae; fi
   # This fails to download sometimes, just retry till it works
-  - while [[ "$TRAVIS_OS_NAME" == "osx" ]] && [[ "$CC" == "gcc" ]] && [[ "$EnableHostOnlyBuild" != "true" ]] && ! type arm-none-eabi-gcc &> /dev/null; do brew install Caskroom/cask/gcc-arm-embedded; done
+  - while [[ "$TRAVIS_OS_NAME" == "osx" ]] && [[ "$CC" == "gcc" ]] && [[ "$EnableHostOnlyBuild" != "true" ]] && ! type arm-none-eabi-gcc &> /dev/null; do brew install armmbed/formulae/arm-none-eabi-gcc; done
 
 # System setup
 install: