Update Secrets.swift to be more _swifty_

- `enum` instead of `@objc class: NSObject`
- No need to `import Foundation`
- Use correct capitalization for "API" acronym

Author: mokagio

.configure

@@ -1,7 +1,7 @@
 {
   "project_name": "WordPressAuthenticator-iOS",
   "branch": "trunk",
-  "pinned_hash": "bec1a27fb1457e77fe36861d02c239590748a970",
+  "pinned_hash": "d71ccb4011793d363fbef513903867dfc672e09a",
   "files_to_copy": [
     {
       "file": "iOS/WordPressAuthenticator-Demo/Secrets.swift",

.configure-files/Secrets.swift.enc

@@ -1,7 +1,4 @@
-import Foundation
-
-@objc
-class ApiCredentials: NSObject {
+enum APICredentials {
 
     // WordPress.com OAuth ClientID / Client Secret
     static let client = "0"

Demo/AuthenticatorDemo/Secrets.example.swift

@@ -8,15 +8,15 @@ extension ViewController {
         // useful to keep it all in one block to show how insanely long it is.
         WordPressAuthenticator.initialize(
             configuration: WordPressAuthenticatorConfiguration(
-                wpcomClientId: ApiCredentials.client,
-                wpcomSecret: ApiCredentials.secret,
+                wpcomClientId: APICredentials.client,
+                wpcomSecret: APICredentials.secret,
                 wpcomScheme: "wordpress-authenticator-ios-demo",
                 wpcomTermsOfServiceURL: "https://wordpress.com/tos/",
                 wpcomBaseURL: "https://wordpress.com",
                 wpcomAPIBaseURL: "https://public-api.wordpress.com/",
-                googleLoginClientId: ApiCredentials.googleLoginClientId,
-                googleLoginServerClientId: ApiCredentials.googleLoginServerClientId,
-                googleLoginScheme: ApiCredentials.googleLoginSchemeId,
+                googleLoginClientId: APICredentials.googleLoginClientId,
+                googleLoginServerClientId: APICredentials.googleLoginServerClientId,
+                googleLoginScheme: APICredentials.googleLoginSchemeId,
                 userAgent: "\(WKWebView.userAgent)-wordpress-authenticator-demo-app",
                 showLoginOptions: true,
                 enableSignUp: true,