Use XmlHTTPRequest in Travis.ajax

There is a bug in firefox, which prevents using `getAllResponseHeaders` for CORS requests. This would not be a problem, because most of the time we use `getResponseHeader`, but unfortunately jQuery's `getResponseHeader` implementation is based on `getAllResponseHeaders`. As far as I know there is no way to refer to original xhr object and use native implementation of `getResponseHeader` (if I'm wrong here, I'll be glad to know how to do it)

The implementation is pretty straightforward, especially because we don't support old browsers, so it seems that we don't need any browser specific hacks (I haven't used XmlHTTPRequest directly for ages, though).