Running akka within an SBT build task: reference configuration unexpectedly missing

I'm trying to run a rather simple HTTP POST request within a task I wrote for my SBT build, and seeing that SBT does not seem to have helpers for that, I settled on spray-client to accomplish this task.

In a project/dependencies.sbt file I put the following:

resolvers += "spray.io repo" at "http://repo.spray.io/"

libraryDependencies ++= Seq(
  "com.typesafe.akka" %% "akka-actor" % "2.2.3",
  "io.spray" % "spray-client" % "1.2.0")

And my task is implemented by:

def uploadSite(version: String, siteArchive: File, log: Logger): HttpResponse = {

  def request: HttpRequest = Post(siteUploadUrl,
    MultipartFormData(Map(
    // abridged
    ))

  implicit val system = ActorSystem() // <-- Exception HERE!
  try {
    import system.dispatcher

    val future: Future[HttpResponse] = pipelining.sendReceive(request)

    Await.result(future, 1 minute)
  }
  finally system.shutdown()
}

The task fails when I run it with the following exception:

com.typesafe.config.ConfigException$Missing: No configuration setting found for key 'akka'
    at com.typesafe.config.impl.SimpleConfig.findKey(SimpleConfig.java:115)
    at com.typesafe.config.impl.SimpleConfig.find(SimpleConfig.java:138)
    at com.typesafe.config.impl.SimpleConfig.find(SimpleConfig.java:150)
    at com.typesafe.config.impl.SimpleConfig.find(SimpleConfig.java:155)
    at com.typesafe.config.impl.SimpleConfig.getString(SimpleConfig.java:197)
    at akka.actor.ActorSystem$Settings.<init>(ActorSystem.scala:136)
    at akka.actor.ActorSystemImpl.<init>(ActorSystem.scala:470)
    at akka.actor.ActorSystem$.apply(ActorSystem.scala:111)
    at akka.actor.ActorSystem$.apply(ActorSystem.scala:93)
    at akka.actor.ActorSystem$.apply(ActorSystem.scala:82)
    at MyIO$.uploadSite(MyIO.scala:65)

My basic analysis is that the reference.conf file that can be found in akka-actor_2.10-2.2.3.jar is not read, for some reason that escapes me and maybe has something to do with how SBT manages its classpath for running the build.

Some precisions: I'm on SBT 0.13.0 (hence Scala 2.10 for the build code) and I've checked that the aforementioned akka-actor jar indeed contains a reference.conf file which is as expected. When looking at what I guess might be related to the build execution classpath (reload plugins then show runtime:fullClasspath in sbt), that jar appears in the list.

I also failed at googling anything relevant, being unable to convey that the problem lies with running akka from within an SBT build.

In the end, I really have no idea how the 'akka' configuration key could be missing. Can anyone help?