fix travis-ci testing
fix travis-ci testing

new elasticsearch-5 package of travis container use x-pack plugin by
default. This introduced the necessity to authenticate all the requests
to the DB.

We need to use the LIBREANT_ES_HOSTS env variable to inform the test
suite about those credentials

insomnia-lab/libreant,insomnia-lab/libreant,insomnia-lab/libreant,insomnia-lab/libreant