Travis - add integration tests with Elasticsearch instance

[macbre]

• https://www.peterbe.com/plog/elasticsearch-5-in-travis-ci
• https://docs.travis-ci.com/user/database-setup/#elasticsearch

https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-oss-6.5.1.tar.gz (33 MB)

env:
  - ES_VERSION=5.1.1 ES_DOWNLOAD_URL=https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-${ES_VERSION}.tar.gz
install:
  - wget ${ES_DOWNLOAD_URL}
  - tar -xzf elasticsearch-${ES_VERSION}.tar.gz
  - ./elasticsearch-${ES_VERSION}/bin/elasticsearch &
script:
  - wget -q --waitretry=1 --retry-connrefused -T 10 -O - http://127.0.0.1:9200

Auto-installed version:

ElasticSearch version
5.5.0

[macbre]

Travis default install:

$ wget -q --waitretry=1 --retry-connrefused -T 10 -O - http://127.0.0.1:9200
{
  "name" : "qUlnzWn",
  "cluster_name" : "elasticsearch",
  "cluster_uuid" : "TcGY5Q9PSXShsdBARWt4nA",
  "version" : {
    "number" : "5.5.0",
    "build_hash" : "260387d",
    "build_date" : "2017-06-30T23:16:05.735Z",
    "build_snapshot" : false,
    "lucene_version" : "6.6.0"
  },
  "tagline" : "You Know, for Search"
}

[macbre]

es-oss v.6.5.1:

$ curl http://127.0.0.1:9200
{
  "name" : "kny8T5d",
  "cluster_name" : "elasticsearch",
  "cluster_uuid" : "f-UrK4-nTPGYuoxM83wnmg",
  "version" : {
    "number" : "6.5.1",
    "build_flavor" : "oss",
    "build_type" : "tar",
    "build_hash" : "8c58350",
    "build_date" : "2018-11-16T02:22:42.182257Z",
    "build_snapshot" : false,
    "lucene_version" : "7.5.0",
    "minimum_wire_compatibility_version" : "5.6.0",
    "minimum_index_compatibility_version" : "5.0.0"
  },
  "tagline" : "You Know, for Search"
}