Update ElasticSearch plugin versions for Travis
Remove array-wrapping from sort attrs for ES 0.90+
Implement safe v1:delete_search_index rake task
Add optional hook for ElasticSearch start-up options

The camp restart script will now source camp##/tmp/.elasticsearch_options
(if it exists) before starting ElasticSearch. This can be used to modify
environment variables on a camp-specific basis.

Example .elasticsearch_options:

export ES_HEAP_SIZE=2g