Upgrade Elasticsearch to 5.5

We are expecting improvements of the search result relevancy by upgrading to Elasticsearch 5.2.

The support for Elasticsearch 2.4 will end on February 28th, 2018. Because the support of our "SonarQube 6.x LTS" version will last longer then this date, we should upgrade Elasticsearch before releasing our LTS (for not to have to backport the Elasticsearch upgrade in February 2018).

From https://www.elastic.co/support/eol:

The lists of breaking changes can be found here https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes.html . We have to keep in mind, that upgrading from 2.4 to 5.4 includes changes from 2 to 5.0 and from 5.0 to 5.4.