Updated to new version of PDF.js library.

This new version should:
- not crash on current Chrome version, see
  https://github.com/mozilla/pdf.js/issues/4104
  https://code.google.com/p/chromium/issues/detail?id=333594
- provide better position of text segments, see
  https://github.com/mozilla/pdf.js/pull/4089
esprima errors when upgrading PDF.js should not happen anymore.

See:
https://github.com/mozilla/pdf.js/issues/4175
https://github.com/mozilla/pdf.js/pull/4180
Document why mrt command occasionally disappears.
Updated PDF.js again.
Updated code to new PDF.js version.