Add requirements.txt to MANIFEST.in

This file is required for Dockerfile builds for a few of the models.

Author: jasonnance

MANIFEST.in

@@ -3,4 +3,5 @@ include README.md
 include meta.json
 # We have some .py files that are needed but aren't part of the package, so make sure they're included
 recursive-include gobbli *.py
-recursive-include gobbli Dockerfile
+recursive-include gobbli Dockerfile
+recursive-include gobbli requirements.txt