Switch away from fork of instantsearch.js

With !2321 (merged) merged, it seemed like a good time to look at if we still need to use @tnir's fork of instantsearch.js.

• Fork: https://github.com/tnir/instantsearch.js
• Upstream: https://github.com/algolia/instantsearch.js

This seems to be the MR where we forked: !1096 (merged).