Bump nltk from 3.4.5 to 3.6.6

Bumps [nltk](https://github.com/nltk/nltk) from 3.4.5 to 3.6.6.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.4.5...3.6.6)

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

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

Author: dependabot[bot]

requirements.txt

@@ -82,7 +82,7 @@ murmurhash==0.28.0
 natsort==5.3.3
 nbconvert==5.4.0
 nbformat==4.4.0
-nltk==3.4.5
+nltk==3.6.6
 nose==1.3.7
 notebook==5.7.8
 numpy==1.16.0