Allow wider definition for _zip?

Some zip files have a different definition, e.g.
Minecraft has:

    application/xml; charset=utf-8 compressed-encoding=application/zip;
      charset=binary; charset=binary

Most other zips have:

    application/x-empty; compressed-encoding=application/zip;
      charset=binary; charset=binary

But the result is the same. So now we test only for:

    compressed-encoding=application/zip; charset=binary; charset=binary

lib/cask.rb

@@ -199,7 +199,7 @@ class Cask
 
   def _zip?(path)
     output = `file -Izb #{path}`
-    output.chomp == 'application/x-empty compressed-encoding=application/zip; charset=binary; charset=binary'
+    output.chomp.include? 'compressed-encoding=application/zip; charset=binary; charset=binary'
   end
 
   def _tar_bzip?(path)