Bump nltk from 3.4.5 to 3.6.6 in /conv_graph

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]

conv_graph/requirements.txt

@@ -7,7 +7,7 @@ jsonschema==2.6.0
 matplotlib==3.1.1
 multidict==4.5.2
 networkx==2.4
-nltk==3.4.5
+nltk==3.6.6
 numpy==1.17.2
 PyYAML==5.4
 regex==2018.1.10