add user agent for Structured Data Testing Tool

Google Structured Data Testing tool: `Google-StructuredDataTestingTool; +http://www.google.com/webmasters/tools/richsnippets`

Author: gagalago

lib/prerender_rails.rb

@@ -23,6 +23,7 @@ def initialize(app, options={})
         'outbrain',
         'pinterest',
         'developers.google.com/+/web/snippet',
+        'www.google.com/webmasters/tools/richsnippets',
         'slackbot',
         'vkShare',
         'W3C_Validator',