Suppress 'brew update' error on travis ci

Author: scenee

.travis.yml

@@ -59,17 +59,11 @@ jobs:
   - stage: "Carthage"
     # Carthage doesn't fix the issue with Xcode 12: https://github.com/Carthage/Carthage/releases/tag/0.36.0
     osx_image: xcode11.6
-    cache:
-      directories:
-      - /usr/local/Homebrew
-      - $HOME/Library/Caches/Homebrew
     before_install:
     - brew update
     - brew outdated carthage || brew upgrade carthage
     script:
     - carthage build --no-skip-current
-    before_cache:
-      - brew cleanup
 
   - stage: "CocoaPods"
     osx_image: xcode11.6