unsigned_kext_req: use new cask/download DSL.

Library/Homebrew/requirements/unsigned_kext_requirement.rb

@@ -5,35 +5,12 @@ class UnsignedKextRequirement < Requirement
 
   satisfy { MacOS.version < :yosemite }
 
-  def initialize(tags=[])
-    tags.each do |tag|
-      next unless tag.is_a? Hash
-      @binary ||= tag[:binary]
-      @cask ||= tag[:cask]
-    end
-    super
-  end
-
   def message
     s = <<-EOS.undent
       Building this formula from source isn't possible due to OS X
       Yosemite and above's strict unsigned kext ban.
     EOS
-
-    if @cask
-      s +=  <<-EOS.undent
-
-        You can install from Homebrew Cask:
-          brew install Caskroom/cask/#{@cask}
-      EOS
-    end
-
-    if @binary
-      s += <<-EOS.undent
-
-        You can use the upstream binary:
-          #{@binary}
-      EOS
-    end
+    s += super
+    s
   end
 end