[JavaScript] Tests JS API client with ES5

##### Description

To catch any issue with ES5 (e.g https://github.com/swagger-api/swagger-codegen/pull/5871), we'll need to setup CI with ES5 with Shippable or CircleCI.

##### Swagger-codegen version

Latest master 

##### Related issues

https://github.com/swagger-api/swagger-codegen/pull/5871

##### Suggest a Fix

If anyone wants to help work on this, please reply to let us know.
