Remove `brew install carthage` from Travis-CI

.travis.yml

@@ -5,8 +5,6 @@ cache:
   - vendor/bundle
 before_install:
 - curl 'https://developer.apple.com/certificationauthority/AppleWWDRCA.cer' -o certificates/AppleWWDRCA.cer
-- brew update; brew update
-- brew install carthage
 install:
 - bundle install --path=vendor/bundle --binstubs=vendor/bin
 - bundle exec rake certificates:install