Swift 5 Support w/ Cocoapods

[jlaws]

• I've read, understood, and done my best to follow the *CONTRIBUTING guidelines.

What did you do?

Built the pod with Xcode 10.2

What did you expect to happen?

No warning to convert Charts.framework to Swift 5

What happened instead?

Xcode created a warning to migration Charts.framework to Swift 5

Charts Environment

Charts version/Branch/Commit Number: Cocoapod version 3.2.2
Xcode version: 10.2
Swift version: 5
Platform(s) running Charts: iOS
macOS version running Xcode: 10.14.3

Demo Project

any project running in Xcode 10.2 will display the warning.

[liuxuan30]

let's route swift 5 PRs to #3874 add anything you found missing in that PR. thanks

[jlaws]

Works great, thanks!