Podspec missing summary and homepage variables [Swift3][Swift4]

Description

The Podspec file in a generated Swift3 or Swift4 client lacks summary and homepage variables, which causes "pod install" to fail.

Swagger-codegen version

2.3.1

Command line used for generation

swagger-codegen generate -i ~/Downloads/swagger-yaml-client-generated/swagger.yaml -l swift4 -o ./server-sdk

Steps to reproduce

Generate a swift3 or swift4 client.
Attempt to integrate the client into an app via Cocoapods. "Pod install" will fail.

$ pod install
Analyzing dependencies
Fetching podspec for `SwaggerClient` from `./server-sdk`
[!] The `SwaggerClient` pod failed to validate due to 2 errors:
    - ERROR | attributes: Missing required attribute `homepage`.
    - ERROR | attributes: Missing required attribute `summary`.
    - WARN  | source: The version should be included in the Git tag.
    - WARN  | source: Git SSH URLs will NOT work for people behind firewalls configured to only allow HTTP, therefore HTTPS is preferred.

Related issues/PRs

This is the same issue as #3844 but for Swift3/Swift4.

Suggest a fix/enhancement

Probably the same as #3850

The generated Podspec for the Swift3/Swift4 client is the entry point; compare its template with the fix suggested in #3850 and related issue #3844. Generate a Swift4 client with the command shown, then run pod install; done when validation no longer reports missing homepage or summary.