Update Podspec.

[renato-anchor]

Summary

The current Podspec is outdated, not reflecting the latest changes in the repository, such as the migration to Swift 5 and the version bump to 5.0.0.

With this pull request, the .podspec will be up to date.

Linter

❯ pod spec lint --allow-warnings

 -> SimpleButton (5.0.0)
    - WARN  | source: The version should be included in the Git tag.
    - NOTE  | xcodebuild:  note: Using new build system
    - NOTE  | xcodebuild:  note: Building targets in parallel
    - NOTE  | xcodebuild:  note: Using codesigning identity override: -
    - NOTE  | xcodebuild:  note: Build preparation complete
    - NOTE  | [iOS] xcodebuild:  note: Planning build
    - NOTE  | [iOS] xcodebuild:  note: Analyzing workspace
    - NOTE  | [iOS] xcodebuild:  note: Constructing build description
    - NOTE  | [iOS] xcodebuild:  warning: Capabilities for Signing & Capabilities may not function correctly because its entitlements use a placeholder team ID. To resolve this, select a development team in the App editor. (in target 'App' from project 'App')
    - NOTE  | [iOS] xcodebuild:  warning: Skipping code signing because the target does not have an Info.plist file and one is not being generated automatically. (in target 'App' from project 'App')

Analyzed 1 podspec.

SimpleButton.podspec passed validation.

Closes: #28