Elasticsearch version: 2.3

OS version: Ubuntu 16.04

Description of the problem including expected versus actual behavior:
Warning from apt-get when installing ElasticSearch (and Logstash): W: http://packages.elastic.co/elasticsearch/2.x/debian/dists/stable/Release.gpg: Signature by key 46095ACC8548582C1A2699A9D27D666CD88E42B4 uses weak digest algorithm (SHA1

Notes:

• It installs just fine, it just warns.
• The same applies to Logstash.

Steps to reproduce:

1. Add elastic GPG key as explained in the documentation
2. Add elasticsearch repository to sources list and update: apt-get update
3. Run apt-get install elasticsearch -y