Logstash version: 2.3.4 (since at least 2.3)

OS version: Ubuntu 16.04.1 (and 16.04)

Description of the problem including expected versus actual behavior:
Warning from apt-get when installing Logstash: W: http://packages.elastic.co/logstash/2.3/debian/dists/stable/Release.gpg: Signature by key 46095ACC8548582C1A2699A9D27D666CD88E42B4 uses weak digest algorithm (SHA1)

Notes:

• It installs just fine, it just warns.

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 logstash -y

See elastic/elasticsearch#17724