Upgraded Elasticsearch to version 5.1.2.

Removed bogus instruction in Dockerfile.

Upgraded Elasticsearch to version 5.1.1.

Fixes pires#23

Print plug-in install.

Upgraded Elasticsearch to version 5.0.1.

Upgraded Elasticsearch to version 5.0.0.

Upgraded Elasticsearch to version 2.4.1.

Upgraded Elasticsearch to 2.4.0. Fixes pires#18

Minimum number of master nodes is now configurable.

Updated instructions related to configuration environment variables.