4.0.0

Major Changes

• Drop node 8 for now: d74ad94
• Replace transport by a new winston 3.x complient one. ES6 style (+ babel and ./dist/ directory containing the compiled version. package.json/main points to ./dist/winston-logstash.js). Reviewed the tests: I still have an error on the end, unable to figure out why for the moment. But the tests are OK.: 8b291f2
• Publication en npm privé en attendant la pull request acceptée: 2c90898
• Update TravisCI config node versions: e9f22d8
• Winston 3 support: #1