Switch to alternative Mathjax CDN

[Wiktor Walc]

​https://www.mathjax.org/cdn-shutting-down/

Actions

1. Verify that the version from cdnjs works
2. Update tests
3. Update API docs source (​http://docs.ckeditor.com/#!/api/CKEDITOR.config-cfg-mathJaxLib)
4. Update SDK source code (​http://sdk.ckeditor.com/samples/mathjax.html)
5. Although we are not introducing any changes (users set the path to a CDN by themselves, following the API docs) we should explain in changelog what's going to happen on May 1st, 2017.

[kkrzton]

Created 3 PRs:

• ​https://github.com/ckeditor/ckeditor-dev/pull/338
• ​https://github.com/ckeditor/ckeditor-sdk/pull/212
• ​https://github.com/ckeditor/ckeditor-docs/pull/86

[Marek Lewandowski]

Fixed with ​git:bae53ff0d38117c12ccde6297a5da24b12121b9b.