use official ES images from Docker Hub (#359)

we sometimes experience timeouts with docker.elastic.co, maybe this
helps

closes #359

Author: beniwohli

tests/docker-compose.yml

@@ -64,7 +64,7 @@ services:
       - "6379:6379"
 
   elasticsearch6:
-    image: docker.elastic.co/elasticsearch/elasticsearch:6.2.0
+    image: elasticsearch:6.5.2
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:9200"]
     environment:
@@ -77,7 +77,7 @@ services:
       - pyesdata6:/usr/share/elasticsearch/data
 
   elasticsearch5:
-    image: docker.elastic.co/elasticsearch/elasticsearch:5.6.0
+    image: elasticsearch:5
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:9200"]
     environment: