Support Elasticsearch v5.1 - v5.5

What does this MR do?

Increases the range of supported Elasticsearch versions from:

v5.1.x - v5.3.x

to:

v5.1.x - v5.5.x

Are there points in the code the reviewer needs to double check?

• Should we change our support to v5.x ?
• Should we drop support for v5.1? 5.2 ?
• Should we run the specs against the earliest version of elasticsearch, or the latest, that we support?

Why was this MR needed?

GitLab.com is currently using v5.4 and an update to v5.5 is scheduled: https://gitlab.com/gitlab-com/infrastructure/issues/2600

Specs pass on v5.4 as well: https://gitlab.com/nick.thomas/gitlab-ee/commit/a808c2481685ca64518c1ed1036b1355a7623f81/pipelines

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-com/infrastructure/issues/2600