Fix regression #4054: use new ajax.getArrayBuffer interface for rtl-text

[ChrisLoer]

There's still no automated testing covering the asynchronous load (because we don't want automated tests dependent on any network interaction). Do we have mocks that would be useful here?

[lucaswoj]

Do we have mocks that would be useful here?

First thing that comes to mind: we could use Sinon's "fake" XMLHttpRequest mock as we do in some unit tests http://sinonjs.org/docs/#server

[anandthakker]

we could use Sinon's "fake" XMLHttpRequest mock

👍 to that -- we technically could use proxyquire to mock the js/util/ajax module, but that doesn't guard against ajax's interface changing, as in this case.