Migrate to Elasticsearch 5.1.* when it's stable enough

Remove requirement for delete_by_query plugin as it's already built-in
Make sure search works as expected (changes to the mapping and some search queries are needed here)
Make sure documents gets deleted when project is deleted (API to delete_by_query functionality has been changed)
Update rake gitlab:app:check task
Requirements
Update the update guide
Check if we really need to rebuild index. Maybe that's enough to migrate index with ES official migrator.