Fix build error on travis-ci

https://github.com/Homebrew/homebrew/issues/42553#issuecomment-128453641

.travis.yml

@@ -5,7 +5,7 @@ cache:
   - vendor/bundle
 before_install:
 - curl 'https://developer.apple.com/certificationauthority/AppleWWDRCA.cer' -o certificates/AppleWWDRCA.cer
-- brew update
+- brew update; brew update
 - brew install carthage
 install:
 - bundle install --path=vendor/bundle --binstubs=vendor/bin