Fix bulk indexing problems with Elasticsearch 1.0

In order for ElasticUtils to work for both Elasticsearch 0.90 and
Elasticsearch 1.0 using elasticsearch-py 0.4.5, we need to do some
monkey-patching of elasticsearch-py.

In this case, calling Elasticsearch.client.bulk() returns an 'ok' field
with ES 0.90 and a 'status' field with ES 1.0. This patch sets the 'ok'
field based on the 'status' field so that the bulk indexing
infrastructure in elasticsearch-py 0.4.5 is testing the right thing and
not raising BulkIndexingErrors.

Fixes #241

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