[CI] Travis: Fix before install

Author: picandocodigo

.travis.yml

@@ -68,7 +68,7 @@ before_install:
   - /tmp/elasticsearch-${ELASTICSEARCH_VERSION}/bin/elasticsearch-keystore create
   - >
     [[ -n "$TEST_SUITE" ]] && [[ "$TEST_SUITE" == "integration" ]] &&
-    TEST_CLUSTER_COMMAND=/tmp/elasticsearch-6.7.0/bin/elasticsearch
+    TEST_CLUSTER_COMMAND=/tmp/elasticsearch-${ELASTICSEARCH_VERSION}/bin/elasticsearch
     bundle exec rake -I elasticsearch-extensions/lib test:cluster:start &&
     curl http://localhost:9250/_cluster/health?level=shards || true