Remove redundant tuple

Resolves a build warning in Xcode 9

DVR/Session.swift

@@ -15,7 +15,7 @@ open class Session: URLSession {
     private var needsPersistence = false
     private var outstandingTasks = [URLSessionTask]()
     private var completedInteractions = [Interaction]()
-    private var completionBlock: ((Void) -> Void)?
+    private var completionBlock: (() -> Void)?
 
     override open var delegate: URLSessionDelegate? {
         return backingSession.delegate
@@ -93,7 +93,7 @@ open class Session: URLSession {
     /// This only needs to be called if you call `beginRecording`. `completion` will be called on the main queue after
     /// the completion block of the last task is called. `completion` is useful for fulfilling an expectation you setup
     /// before calling `beginRecording`.
-    open func endRecording(_ completion: ((Void) -> Void)? = nil) {
+    open func endRecording(_ completion: (() -> Void)? = nil) {
         if !recording {
             return
         }