Use image_path instead of asset_path in readme

In Rails 4.2 `#asset_path` has changed. Prior to Rails 4.2,
`#asset_path` would search all asset directories, and prepend the
appropriate asset type directory name to the result:

  asset_path("emoji/unicode/<id>.png")
  #=> "/images/emoji/unicode/<id>.png"

However, In Rails 4.2 `#asset_path` doesn't do that anymore, and assumes
that you've specified the correct folder. Since Emoji images are copied
to public/images/emoji/unicode, `#asset_path` would then generate the
wrong URL path:

  asset_path("emoji/unicode/<id>.png")
  #=> "/emoji/unicode/<id>.png"

Using `#image_path` fixes that, because that method looks specifically
for images/ folder, so it will find the Emoji images.

README.md

@@ -57,7 +57,7 @@ module EmojiHelper
   def emojify(content)
     h(content).to_str.gsub(/:([\w+-]+):/) do |match|
       if emoji = Emoji.find_by_alias($1)
-        %(<img alt="#$1" src="#{asset_path("emoji/#{emoji.image_filename}")}" style="vertical-align:middle" width="20" height="20" />)
+        %(<img alt="#$1" src="#{image_path("emoji/#{emoji.image_filename}")}" style="vertical-align:middle" width="20" height="20" />)
       else
         match
       end

test/documentation_test.rb

@@ -18,7 +18,7 @@ class DocumentationTest < TestCase
       str.gsub('<', '&lt;').gsub('>', '&gt;')
     end
 
-    def self.asset_path(img)
+    def self.image_path(img)
       "/images/#{img}?123"
     end
   end