could not find implicit value for parameter marshaller

build.sbt

name := """short-url"""

version := "1.0"

scalaVersion := "2.11.2"

libraryDependencies ++= Seq(
  "io.spray" % "spray-can" % "1.3.1" % "compile",
  "io.spray" % "spray-routing" % "1.3.1" % "compile",
  "com.typesafe.akka" %% "akka-actor" % "2.3.3"
)

gistfile1.scala

package core

import akka.actor.ActorSystem
import spray.routing.{Route, SimpleRoutingApp, ValidationRejection}

import scala.concurrent.Future
import scala.util.Random

/**
 *
 */
object ShortUrl extends App with SimpleRoutingApp {

  implicit val actorSystem = ActorSystem("Short-Url")
  startServer("0.0.0.0", 8080)(route)


  implicit def executionContext = actorRefFactory.dispatcher

  def getExampleBooleanResponse : Boolean = {
      Random.nextBoolean()
  }
  def getExampleStringResponse : Option[String] = {
    Some(Random.alphanumeric.take(7).mkString)
  }
  def route = Route {
    path("shorten") {
      post { ctx =>
        val r = ctx.request.entity.asString
        val f = Future(getExampleBooleanResponse)

        f.mapTo[Boolean].map {
          case true => ctx.complete {
            s"http://localhost:8080/"
          }
          case _ => ctx.reject(ValidationRejection(s"Cannot save url $r"))
        }
      }
    }~
    path(Rest) { r =>
      get { ctx =>
        Future(getExampleStringResponse).mapTo[Option[String]].map {
          case Some(result) => ctx.complete(result)
        }
      }
    }

  }
}