fix(podspec): specify swift_versions array supporting Swift 4.0, 4.2, and 5.0 - #169

Summary

Updates MagazineLayout.podspec to specify s.swift_versions = ['4.0', '4.2', '5.0'] instead of pinning single version '4.0'.

Context & Motivation

Fixes #111.

The framework supports Swift 5 (and defines swiftLanguageVersions: [.v5] in Package.swift). However, MagazineLayout.podspec previously set s.swift_version = '4.0', causing CocoaPods integration errors when integrating apps specify supports_swift_versions '>= 5.0':
\
[!] Unable to determine Swift version for the following pods:

• \MagazineLayout\ does not specify a Swift version (\4.0) that is satisfied by any of targets integrating it.
  \\

By declaring s.swift_versions = ['4.0', '4.2', '5.0'], CocoaPods can properly resolve integration across Swift 4.x and Swift 5.x targets.

Verification

• Verified podspec syntax against CocoaPods Podspec specification for \swift_versions.
• Confirmed alignment with \Package.swift's Swift 5 support.