upgrade akka version (#100)

- tested with akka-http application

Author: Christophe Pache

project/Build.scala

@@ -7,7 +7,7 @@ object ScalaOAuth2Build extends Build {
   lazy val _organization = "com.nulab-inc"
   lazy val _version =  "0.17.3-SNAPSHOT"
   lazy val _playVersion = "2.5.0"
-  lazy val akkaV = "2.4.2"
+  lazy val akkaV = "2.4.7"
   val _scalaVersion = "2.11.7"
   val _crossScalaVersions = Seq("2.11.7")