Adapt to support PDF.js v1.0.907

... as per #86.

Tested with:
 * Iceweasel 31.2.0 (Shipped with Debian, built on top of Firefox ESR) - ships with PDF.js v1.0.68
 * Firefox 31.3.0 (current ESR a.k.a. Extended Support Release) - ships with PDF.js v1.0.68
 * Firefox 34.0.5 (current stable) - ships with PDF.js v1.0.712
 * Firefox 34.0.5 (current beta - yes, it's the same as stable, just released) - ships with PDF.js v1.0.712
 * Firefox 35.0a2 (developer edition a.k.a. aurora a.k.a. alpha) - ships with PDF.js v1.0.907

According to tests, all involved PDF.js versions are now fully supported.

> The methods themselves will have the scope of the object, unless they are passed around as function values, such as for callbacks or event binding. [...] You do need it on the Promise callback, [...]

We are calling this method from a Promise callback, [here](https://github.com/hypothesis/annotator/pull/88/files#diff-72e265f58fee2312856ad92bd12f37cbR119).

Because you insisted, I tried with a thin arrow. It does not work.