Run brew update in before_install

In older versions of Homebrew, such as the one pre-installed on the
Travis OSX 10.9 image, `brew update` will not checkout the previously
checked-out commit in a tap after pulling the latest master.

ci/travis/before_install.sh

@@ -52,6 +52,8 @@ run ruby --version
 run which gem
 run gem --version
 
+run brew update
+
 # ensure that we are the only brew-cask available
 run brew uninstall --force brew-cask
 

ci/travis/install.sh

@@ -22,7 +22,6 @@ if must_run_tests; then
   run bundle install --path="${GEM_HOME%/*/*}" --with=test
 
   # install Formulae and Casks without which some tests would be skipped
-  run brew update
   brew_install cabextract
   brew_install unar
   run brew cask install Casks/adobe-air.rb