silent html5 validation problems

- deprecation notice of w3 service causes build to fail (upstream issue: https://github.com/praveenvijayan/grunt-html-validation/issues/76)
- should ampersand be escaped in url parameters? opinions differ; current practice in docs is not to escape URL parameter ampersands, and this need not cause the build to fail

Gruntfile.js

@@ -276,7 +276,9 @@ module.exports = function (grunt) {
         reset: true,
         relaxerror: [
           'Bad value X-UA-Compatible for attribute http-equiv on element meta.',
-          'Element img is missing required attribute src.'
+          'Element img is missing required attribute src.',
+          'This interface to HTML5 document checking is deprecated.',
+          '\\& did not start a character reference. \\(\\& probably should have been escaped as \\&.\\)'
         ]
       },
       files: {