build(deps): bump nltk from 2.0.5 to 3.4.5

Bumps [nltk](http://nltk.org/) from 2.0.5 to 3.4.5.

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

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 Flask==0.12
 MarkupSafe==1.0
-nltk==2.0.5
+nltk==3.4.5
 nose==1.3.0
 pymongo==2.6
 bashlex==0.12