Upgrade elasticsearch 0 20 6

This pull request contains a few things:

1. The query has been re-written to make use of some of the power of 0.20.6. See this commit for fuller explanation:
https://github.com/alphagov/rummager/commit/e44e9462c4b1d3d8bc33ef63fbaf63619e0badaa

2. Uses a different stemmer

3. Removes field weightings

4. Different way of doing shingle queries, which is clearer

5. Introduces more awesome