Mac Version doesn't compile

[Vithanco]

I might overlook something, but the a8c1dfd version doesn't seem to compile on a Mac. UseStoreKitReviewPrompt seems to be missing. It is only declared for iOS.

#if os(iOS)
    public func useStoreKitReviewPrompt() -> Bool {
...

but it is needed elsewhere, like in shouldTryStoreKitReviewPrompt, where it says function is available for all macOS versions.

    public var shouldTryStoreKitReviewPrompt : Bool {
        if #available(iOS 10.3, *), useStoreKitReviewPrompt { return true }

Please review. Thanks for this great library.

[Vithanco]

I don't think this issue was introduced with a8c1dfd.

[coneybeare]

It was introduced on #112 by @MartinMoizard, but I haven't had time to revert it. Pulls welcome.