🐛 Marks RxViewModel as open

When `RxViewModel` was created `Swift` only had 3 access levels; and
thus marking it as `public` would suffice for subclassing; with `Swift`
3 this changed and we never got around modifying the access level on the
class to be `open` as reported on #50.

Source/RxViewModel.swift

@@ -16,7 +16,7 @@ import RxSwift
 Implements behaviors that drive the UI, and/or adapts a domain model to be 
 user-presentable.
 */
-public class RxViewModel: NSObject {
+open class RxViewModel: NSObject {
   // MARK: Constants
   let throttleTime: TimeInterval = 2