Bump nltk from 3.4.1 to 3.6.6

Bumps [nltk](https://github.com/nltk/nltk) from 3.4.1 to 3.6.6.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](https://github.com/nltk/nltk/compare/3.4.1...3.6.6)

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,4 +1,4 @@
-nltk==3.4.1
+nltk==3.6.6
 gensim==3.7.2
 tqdm==4.31.1
 numpy==1.16.3