Update word tokenizer

Summary:
Switch to the treebank tokenizer to cut a sentence into words.

PunktWordTokenizer is now considered as an internal part of punkt,
and not a public API class.

See https://github.com/nltk/nltk/commit/0b91a7160717faa2fe93d42f7c6bba735f6dd48a

Test Plan: Tested with Ada Palmer, the Will to Battle

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1353