Update nltk to 3.4

[pyup-bot]

This PR updates nltk from 3.3 to 3.4.

Changelog

3.4

* Support Python 3.7	
* Language Modeling incl Kneser-Ney, Witten-Bell, Good-Turing
* Cistem Stemmer for German
* Support Russian National Corpus incl POS tag model
* Decouple sentiment and twitter packages
* Minor extensions for WordNet
* K-alpha
* Fix warning messages for corenlp
* Comprehensive code cleanups
* Many other minor fixes
* Switch continuous integration from Jenkins to Travis

Special thanks to Ilia Kurenkov (Language Model package), Liling Tan (Python 3.7, Travis-CI),
and purificant (code cleanups). Thanks also to: Afshin Sadeghi, Ales Tamchyna, Alok Debnath,
aquatiko, Coykto, Denis Kataev, dnc1994, Fabian Howard, Frankie Robertson, Iaroslav Tymchenko,
Jayakrishna Sahit, LBenzahia, Leonie Weißweiler, Linghao Zhang, Rohit Kumar, sahitpj,
Tim Gianitsos, vagrant, 53X

Links

• PyPI: https://pypi.org/project/nltk
• Changelog: https://pyup.io/changelogs/nltk/
• Homepage: http://nltk.org/