(ElasticSearch 1.1) Support ES 1.1

* Scalar fields are now single-element arrays
* Removed "Related Content" search
* "Mapping Exists API" now returns empty object instead of error
(ElasticSearch 1.1) Remove manual install of ES 0.90 from .travis.yml
(Elasticsearch 1.1) Added better explanation about search document model
(ElasticSearch 1.1) Split _.extend operations into multiple statements
Merge pull request #886 from mrvisser/elasticsearch-1.1

(ElasticSearch 1.1) Support ES 1.1