Use empty tuple instead of Void in Task to support Swift 4.

Author: nlutsenko

Sources/BoltsSwift/Task.swift

@@ -93,7 +93,7 @@ public final class Task<TResult> {
  }
 
  class func emptyTask() -> Task<Void> {
- return Task<Void>(state: .success())
+ return Task<Void>(state: .success(()))
  }
 
  // MARK: Execute
@@ -107,7 +107,7 @@ public final class Task<TResult> {
  - parameter closure: The closure that returns the result of the task.
  The returned task will complete when the closure completes.
  */
- public convenience init(_ executor: Executor = .default, closure: @escaping ((Void) throws -> TResult)) {
+ public convenience init(_ executor: Executor = .default, closure: @escaping (() throws -> TResult)) {
  self.init(state: .pending())
  executor.execute {
  self.trySet(state: TaskState.fromClosure(closure))
@@ -123,7 +123,7 @@ public final class Task<TResult> {
 
  - returns: A task that will continue with the task returned by the given closure.
  */
- public class func execute(_ executor: Executor = .default, closure: @escaping ((Void) throws -> TResult)) -> Task {
+ public class func execute(_ executor: Executor = .default, closure: @escaping (() throws -> TResult)) -> Task {
  return Task(executor, closure: closure)
  }