Bump nltk from 3.4.5 to 3.6.5

Bumps [nltk](https://github.com/nltk/nltk) from 3.4.5 to 3.6.5.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.4.5...3.6.5)

---
updated-dependencies:
- dependency-name: nltk
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -6,6 +6,6 @@ scipy==0.18.1
 Werkzeug==0.15.3
 gunicorn==19.7.1
 vaderSentiment==2.5
-nltk==3.4.5
+nltk==3.6.5
 textstat==0.4.1
 requests==2.20.0