T4446: Fix Elasticsearch support for 1.0 and above

Summary:
Elasticsearch 1.0 deprecated the "filter" top-level
parameter in favor of "post_filter" which is applied
after scores and so forth are calculated.

Instead search field.corpus with a term query.

Test Plan:
Tested against Elasticsearch 1.1.1, able to perform
basic queries without query parse errors.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4446

Differential Revision: https://secure.phabricator.com/D9321