I deleted some of .rb files(cassandra.rb,cassandra@2.1.rb and cassandra@2.2.rb) from folder /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formul
/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formul
Install directly from GitHub as mentioned here - How to restore brew formula after I rm it
brew install https://github.com/Homebrew/homebrew-core/tree/master/Formula/cassandra.rb