DSP-11503 downgrade akka

newer akka causes problems when assembled into fatjar.

The error is:

[2016-12-19 15:33:52,515] ERROR o.a.s.d.DseSparkSubmitBootstrapper [] [] - Failed to start or submit Spark application
java.lang.VerifyError: Inconsistent stackmap frames at branch target 152
Exception Details:
  Location:
    akka/dispatch/Mailbox.processAllSystemMessages()V @152: getstatic
  Reason:
    Type top (current frame, locals[9]) is not assignable to 'akka/dispatch/sysmsg/SystemMessage' (stack map, locals[9])
(...)

Author: jtgrabowski

project/Versions.scala

@@ -4,7 +4,7 @@ object Versions {
   lazy val metrics = "2.2.0"
   lazy val jodaTime = "2.9.3"
   lazy val jodaConvert = "1.8.1"
-  lazy val akka = "2.4.9"
+  lazy val akka = "2.3.15"
   lazy val spray = "1.3.3"
   lazy val sprayJson = "1.3.2"
   lazy val spark = "2.0.2"