Allow unvoting

So you're gonna wanna do something like:

```ruby
require 'policy_helper'

describe VotePolicy do
  subject { VotePolicy }
    permissions :destroy do
        it "user can destroy their own vote" do
          user = FactoryGirl.create(:user)
          vote = FactoryGirl.create(:vote, user: user)
          expect(subject).to permit(user, vote)
      end

        it "user can destroy another user's vote" do
          user = FactoryGirl.create(:user)
          other_user = FactoryGirl.create(:user)
          vote = FactoryGirl.create(:vote, user: other_user)
          expect(subject).not_to permit(user, vote)
      end
    end
  end
end
```