Update a release blog post and an update guide about Elasticsearch 5.1
We need to leave a note in an update guide about updating Elasticsearch to 5.1.x Also the blog post should be updated as well.
Proposed text for an update guide: GitLab 9.0 needs Elasticsearch to be updated to 5.1.x. Please update it following official documentation. If you have several nodes in a cluster the update can be done without any downtime. Indexes created by Elasticsearch 2.4.x can be read by Elasticsearch 5.1.x.