build(deps): 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

@@ -1,5 +1,5 @@
 Flask==0.12
-nltk==3.4.5
+nltk==3.6.6
 MarkupSafe==1.1.1
 nose==1.3.0
 pymongo==2.6