[PHP][Symfony] Enhancements to Symfony generator

Description

Here are some enhancements for PHP Symfony generator:

• Use camelize instead of initialCaps
• Use File.separator instead of "/" to make it work in the Windows platform
• Better README with installation instructions, how to start, etc
• Setup Symfony Petstore integration tests in the CI (e.g. Travis, Shippable, etc)
• Autogenerate Dockerfile

Please feel free to reply if you've any other suggestions.

Swagger-codegen version

Latest master

Related issues

https://github.com/swagger-api/swagger-codegen/pull/5973

Suggest a Fix

If anyone wants to work on these enhancements, please reply to let us know.

cc @ksm2