Merge pull request #6383 from Bargs/engine50

Bump target ES version to 5.0.0 to stay in sync with ES master

src/plugins/elasticsearch/index.js

@@ -29,7 +29,7 @@ module.exports = function ({ Plugin }) {
           key: string()
         }).default(),
         apiVersion: Joi.string().default('master'),
-        engineVersion: Joi.string().valid('^3.0.0').default('^3.0.0')
+        engineVersion: Joi.string().valid('^5.0.0').default('^5.0.0')
       }).default();
     },