Prevent anon/banned/member users from voting (fix #2719).

There was a regression in 6d6d00b; `before_filter :voter_only` was a
no-op in the post vote controller because it merely returned false,
which does not halt the request. The fix is to arrange for a voter_only
method to be defined that properly redirects to the access denied page.

app/controllers/post_votes_controller.rb

@@ -14,10 +14,4 @@ class PostVotesController < ApplicationController
   rescue PostVote::Error => x
     @error = x
   end
-
-protected
-
-  def voter_only
-    CurrentUser.is_voter?
-  end
 end