Bump elasticsearch to 0.20.2.

The bump from 0.19 to 0.20.1 in https://github.com/travis-ci/travis-cookbooks/commit/96b3b6fe0a3598de72f71b80f4a97cb0c475534d breaks one of my builds. 

This is an elasticsearch bug that I've confirmed has been fixed in [0.20.2](http://www.elasticsearch.org/download/2012/12/27/0.20.2.html), and I'd much appreciate an update.