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

.travis.yml

@@ -7,7 +7,7 @@ services:
   - elasticsearch
 
 env:
-    - ES_VERSION=1.x
+  - LIBREANT_ES_HOSTS="http://elastic:changeme@localhost:9200"
 
 matrix:
     fast_finish: true