Bump nltk from 3.4.5 to 3.6.6 in /scripts/code_summarization

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]

scripts/code_summarization/requirements.txt

@@ -1,4 +1,4 @@
-nltk==3.4.5
+nltk==3.6.6
 numpy==1.15.4
 scikit_learn==0.20.2
 torch==1.0.0