Update pdf.js to 4834f1c

Fixes https://github.com/izuzak/atom-pdf-view/issues/18. 

I think that performance is worse than before -- not sure if that's related to pdf.js or to Atom's upgrade to Chrome 36 which happened with version 0.121.0. Tried investigating, but didn't find anything that I could do to improve performance. Will open another issue to track this.