Update Yarn documentation (0.18.1)

Fixes #545.

lib/docs/filters/yarn/clean_html.rb

@@ -43,6 +43,10 @@ module Docs
           end
         end
 
+        css('.__cf_email__').each do |node|
+          node.replace(decode_cloudflare_email(node['data-cfemail']))
+        end
+
         css('div.highlighter-rouge').each do |node|
           node['data-language'] = node['class'][/language-(\w+)/, 1] if node['class']
           node.content = node.content.strip

lib/docs/scrapers/yarn.rb

@@ -1,7 +1,7 @@
 module Docs
   class Yarn < UrlScraper
     self.type = 'yarn'
-    self.release = '0.17.6'
+    self.release = '0.18.1'
     self.base_url = 'https://yarnpkg.com/en/docs/'
     self.links = {
       home: 'https://yarnpkg.com/',