Remove NLTK corpus from repository; download it from S3 instead

Addresses issue #49 

As far as I can tell, the NLTK data was being used only by the test suite.  In the Ansible deploy scripts, we download the corpus from S3.  I've updated the Travis scripts here so they also download from S3, which ensures that the test suite will use the exact same corpus as prod (or at least, what we'll soon be using in prod once the VPC work lands).

@singingwolfboy