Downgrade Scala to v2.11.8

Scala runtime is incompatible between v2.12 and v2.11.
Because Spark requires v2.11, it is way better to align all the plugins
related to the Analytics to the lower version.

Change-Id: I4613c787d2d54488d716805c4e47259fa8d90c67
diff --git a/build.sbt b/build.sbt
index fc26036..14857fe 100644
--- a/build.sbt
+++ b/build.sbt
@@ -9,7 +9,7 @@
   .settings(
     name := pluginName,
 
-    scalaVersion := "2.12.5",
+    scalaVersion := "2.11.8",
 
     libraryDependencies ++= Seq(
       "com.google.inject" % "guice" % "3.0" % Provided,