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.

In other words, if this is being called as `pdfTextMapper._waitForInit()` then it already has the right scope. Only the inner closures for `tryIt` and `setTimeout` need the fat arrow. Notice how you don't have it on `scan`. You do need it on the `Promise` callback, so that you can call `@_waitForInit`, but at that point you're invoking it as a property on the `this` Object, so it doesn't need a fat arrow.