fix(rxjs): Correct accessibility issues with memorizing subject

A new version of RXJS marked these as public and not protected. 

See: ReactiveX/rxjs#3544

package.json

@@ -58,6 +58,6 @@
   },
   "dependencies": {
     "eventemitter3": "^2.0.3",
-    "rxjs": "^5.5.6"
+    "rxjs": "^5.5.8"
   }
 }

src/participant.ts

@@ -307,7 +307,7 @@ export class Participant extends EventEmitter {
    * the RPC system.
    */
   private attachListeners() {
-    this.rpc = new RPC(this.frame.contentWindow, '1.0');
+    this.rpc = new RPC(this.frame.contentWindow!, '1.0');
 
     this.rpc.expose('sendInteractivePacket', data => {
       this.websocket!.send(

src/reactive.ts

@@ -42,7 +42,7 @@ export class MemorizingSubject<T> extends Subject<T> {
   }
 
   // tslint:disable-next-line
-  protected _subscribe(subscriber: Subscriber<T>): Subscription {
+  _subscribe(subscriber: Subscriber<T>): Subscription {
     const subscription = super._subscribe(subscriber);
     if (subscription && this.hasSet && !(<ISubscription>subscription).closed) {
       subscriber.next(this.value);