Bump nltk from 3.6.5 to 3.6.6 in /neuroscout

Bumps [nltk](https://github.com/nltk/nltk) from 3.6.5 to 3.6.6.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.6.5...3.6.6)

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

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

neuroscout/requirements.txt

@@ -41,7 +41,7 @@ MarkupSafe==1.0
 marshmallow==3.7.1
 moviepy==0.2.3.5
 nibabel==2.1.0
-nltk==3.6.5
+nltk==3.6.6
 numpy==1.18.0
 olefile==0.44
 packaging==16.8