Fix compatibility with `$.noConflict(true)`

`window.jQuery` → `jQuery`

Rationale is well described here in [Bootstrap issue](https://github.com/twbs/bootstrap/issues/10038)

TL;DR: `window.jQuery` breaks browserify/webpack/etc. Just `jQuery` doesn't.

Notes:

- it's a copy of #f140084f2beb17b0a266b42ab9a03d0b935e5f66 (from Bootstrap)
- it should not break #58bfb2248b83098115422ef901ae58c6f8c2169b (from Fotorama)