[emoji-filter] Override emoji_image_tag

This is nicer because I want to use everything that EmojiFilter
does *except* the image tag generation.

The prior implementation had to get involved with finding emojis and
gsubbing them out.

This possible now because of @kbrock's change over here:

gjtorikian/html-pipeline#195

lib/html/pipeline/reveal_ck_emoji_filter.rb

@@ -4,13 +4,8 @@ class Pipeline
     # width/height/align attributes so that reveal-ck supplied CSS
     # takes effect.
     class RevealCKEmojiFilter < EmojiFilter
-      def emoji_image_filter(text)
-        return text unless text.include?(':')
-
-        text.gsub(emoji_pattern) do
-          name = Regexp.last_match[1]
-          "<img class='emoji' alt=':#{name}:' src='#{emoji_url(name)}' />"
-        end
+      def emoji_image_tag(name)
+        "<img class='emoji' alt=':#{name}:' src='#{emoji_url(name)}' />"
       end
     end
   end