* Add build script and remove babel
This change adds a build script which uses babel to transpile generated es6 source to es5, such that it can then be used in node projects or published as a package to npm. The `babel` package is also removed as it is deprecated, and superseded by `babel-cli`.
See https://github.com/OpenAPITools/openapi-generator/issues/2126#issuecomment-473926663
* update samples
* add multiple servers support to JS ES6
* multiple server support in js es5
* using exports in es5
* fix index check
* add oas v3 js es6 client to travis
