Fix: hosts => "es_host:port" regression

[kares]

Same as: logstash-plugins/logstash-filter-elasticsearch#156

The change here makes sure "es_host:port" is properly split into a { host: es_host, port: port } pair.
Elasticsearch gem not seeing a port: key will add the port: 9200 default and Manticore 0.8 appends the port part while building the URL.

The fix is demonstrated by the SECURE_INTEGRATION=true targets :

• 🔴 before (the updated setup_hosts): https://app.travis-ci.com/github/logstash-plugins/logstash-input-elasticsearch/builds/246624077
• 🟢 after (updated setup_hosts): https://app.travis-ci.com/github/logstash-plugins/logstash-input-elasticsearch/builds/246624839

Also added some explicit CI targets against Manticore 0.7.1 which has been dominant through 7.x line.

---

Unfortunately specs are 🔴 on CI due various issues:

• custom user agent no longer set since LS 7.16 #167
• Test failing with JNR error: getprotobyname_r failed #165
• [META] problems surfacing due switching docker base image elastic/logstash#13572

[jsvd]

LGTM