Use unpkg to get a proper HTTPS link to CSSLint

Issue codemirror#5356

Author: marijnh

demo/lint.html

@@ -11,7 +11,7 @@
 <script src="../mode/css/css.js"></script>
 <script src="//cdnjs.cloudflare.com/ajax/libs/jshint/2.9.5/jshint.min.js"></script>
 <script src="//rawgithub.com/zaach/jsonlint/79b553fb65c192add9066da64043458981b3972b/lib/jsonlint.js"></script>
-<script src="//csslint.net/js/csslint.js"></script>
+<script src="https://unpkg.com/csslint@1.0.5/dist/csslint.js"></script>
 <script src="../addon/lint/lint.js"></script>
 <script src="../addon/lint/javascript-lint.js"></script>
 <script src="../addon/lint/json-lint.js"></script>