Update README.adoc

brew  cast changes the repo.

README.adoc

@@ -49,7 +49,8 @@ You need to have http://brew.sh/[homebrew] installed and ensure your version of
 
 [source,bash]
 ----
-brew install caskroom/cask/brew-cask
+brew tap caskroom/cask
+brew install brew-cask
 brew install redis homebrew/versions/cassandra21 packer
 brew cask install java
 cd $SPINNAKER_HOME