[Swift4] Update response wrapper when using PromiseKit wrapper

Description

When using the swagger-codegen with the "responseAs": "PromiseKit" option to wrap the API responses, the generated code is not compatible with the PromiseKit framework version set in the Cartfile created by the generator.

Swagger-codegen version

2.3.1

Command line used for generation

swagger-codegen generate -i <swagger.json file> -l swift4 -c <config.json file> -o generatedFiles/

Content of Cartfile created by the generator

github "Alamofire/Alamofire" ~> 4.5.0
github "mxcl/PromiseKit" ~> 4.4

Content of config.json file

{
    "responseAs": "PromiseKit"
}

Generated code sample

open class func someMethod( someParam: String) -> Promise<[String]> {
    let deferred = Promise<[String]>.pending()
    someMethod(someParam: String) { data, error in
        if let error = error {
            deferred.reject(error) <-- Value of tuple type '(promise: Promise<[String]>, resolver: Resolver<[String]>)' has no member 'reject'
        } else {
            deferred.fulfill(data!) <-- Value of tuple type '(promise: Promise<[String]>, resolver: Resolver<[String]>)' has no member 'fulfill'
        }
    }
    return deferred.promise
}

Suggest a fix/enhancement

The generated code should be:

open class func someMethod( someParam: String) -> Promise<[String]> {
    let deferred = Promise<[String]>.pending()
    someMethod(someParam: String) { data, error in
        if let error = error {
            deferred.resolver.reject(error)
        } else {
            deferred.resolver.fulfill(data!)
        }
    }
    return deferred.promise
}

Reproduce the Swift4 generation command with responseAs set to PromiseKit and compare the generated wrapper with the Swift code shown in the issue. Trace the Swift4 generator template responsible for the response wrapper, then verify that generated code uses the resolver API required by the PromiseKit version in the generated Cartfile.