title: brew cask uninstall --force virtual box gives Error: undefined method `<=' for nil:NilClass
body: <p>On OSX El Captain 10.11.6, I have </p>
  
  <pre><code>$ VirtualBox --help
  Oracle VM VirtualBox Manager 5.0.16
  </code></pre>
  
  <p>I'm trying to uninstall <code>virtual box</code> but getting following ...</p>
  
  <pre><code>brew cask uninstall --force virtualbox
  Error: undefined method `&lt;=' for nil:NilClass
  Did you mean?  &lt;=&gt;
  Follow the instructions here:
    https://github.com/caskroom/homebrew-cask#reporting-bugs
  /usr/local/Caskroom/virtualbox/.metadata/5.0.16-105871/20160414170649.906/Casks/virtualbox.rb:2:in `block in load'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask.rb:23:in `instance_eval'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask.rb:23:in `initialize'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask_loader.rb:29:in `new'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask_loader.rb:29:in `cask'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask_loader.rb:65:in `cask'
  /usr/local/Homebrew/Library/Homebrew/compat/hbc/cask_loader.rb:10:in `cask'
  /usr/local/Caskroom/virtualbox/.metadata/5.0.16-105871/20160414170649.906/Casks/virtualbox.rb:1:in `load'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask_loader.rb:55:in `instance_eval'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask_loader.rb:55:in `load'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cask_loader.rb:168:in `load'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cli/uninstall.rb:19:in `block in run'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cli/uninstall.rb:12:in `each'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cli/uninstall.rb:12:in `run'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cli/abstract_command.rb:35:in `run'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cli.rb:98:in `run_command'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cli.rb:168:in `run'
  /usr/local/Homebrew/Library/Homebrew/cask/lib/hbc/cli.rb:132:in `run'
  /usr/local/Homebrew/Library/Homebrew/cmd/cask.rb:8:in `cask'
  /usr/local/Homebrew/Library/Homebrew/brew.rb:100:in `&lt;main&gt;'
  </code></pre>
  
  <p>Here is first line of the file it is complaining about :</p>
  
  <p><code>cat /usr/local/Caskroom/virtualbox/.metadata/5.0.16-105871/20160414170649.906/Casks/virtualbox.rb</code></p>
  
  <pre><code>cask 'virtualbox' do
    if MacOS.release &lt;= :lion
  </code></pre>
  
  <p>Any ideas ?</p>