Allow conversion of Dependencies to Array

This is needed for the intersection code in `brew deps` to work right,
but can hopefully be refactored away somehow eventually.

Library/Homebrew/dependencies.rb

@@ -108,6 +108,10 @@ class Dependencies
   def *(arg)
     @deps * arg
   end
+
+  def to_ary
+    @deps
+  end
 end
 
 module Dependable