fixing bug with full casks listing introduced recently

Full package listing is broken with some recent changes so fixing that.


    brew cask search
    class Cask::CLI::Search
    Error: private method `gsub' called for nil:NilClass
    Please report this bug:
        https://github.com/Homebrew/homebrew/wiki/troubleshooting
    /usr/local/Cellar/brew-cask/0.27.0/rubylib/cask/cli/search.rb:14:in `run'
    /usr/local/Cellar/brew-cask/0.27.0/rubylib/cask/cli.rb:36:in `process'
    /usr/local/bin/brew-cask.rb:6
    /usr/local/Library/brew.rb:67:in `require'
    /usr/local/Library/brew.rb:67:in `require?'
    /usr/local/Library/brew.rb:113