The following code would work:

it "updates its cumulative score when a comment gets a score" do
  comment_1 = @link.comments.first # put the first comment created by the before block into a variable
  comment_1.vote_up
       
  comment_2 = FactoryGirl.create(:comment, link:@link)
  2.times { comment_2.vote_up }
  
  @link.score.should == 5
end