Skip over doc_count_error_upper_bound when deserializing aggregations

Deserializing a search response with bucket aggregations in it fails because the `doc_count_error_upper_bound` is unexpected. Take this response snippet for example:

`... "aggregations":{"myBucket":{"doc_count_error_upper_bound":105322,"buckets":[{"key":"00000000-1111-1111-0000-070000000000","doc_count":202391...`

This commit skips over this value and lets the method continue as intended. This has been tested on `elasticsearch 1.4.0.beta1`.

Without this change I'm unable to use bucket aggregations.