Merge pull request #333 from ReactiveCocoa/scheduler-class-bound

Give `Scheduler` a class bound.

Author: mdiep

Sources/Scheduler.swift

@@ -14,7 +14,7 @@ import Foundation
 #endif
 
 /// Represents a serial queue of work items.
-public protocol Scheduler {
+public protocol Scheduler: class {
 	/// Enqueues an action on the scheduler.
 	///
 	/// When the work is executed depends on the scheduler in use.