assemblyMergeStrategy type error

I am trying to set up a small project to have an aws-lambda written in scala:

javacOptions ++= Seq("-source", "1.8", "-target", "1.8", "-Xlint")

lazy val root = (project in file(".")).
  settings(
    name := "xxx",
    version := "0.1",
    scalaVersion := "2.12.3",
    retrieveManaged := true
  )

libraryDependencies ++= Seq(
  "com.amazonaws" % "aws-lambda-java-core"   % "1.1.0" % Provided,
  "com.amazonaws" % "aws-lambda-java-events" % "1.1.0" % Provided,
  "org.scalatest" % "scalatest" % "2.2.6" % Test
)

scalacOptions += "-deprecation"

assemblyMergeStrategy in assembly <<= (assemblyMergeStrategy in assembly) {
  (old) => {
    case PathList("META-INF", xs @ _*) => MergeStrategy.discard
    case x => MergeStrategy.first
  }
}

Results in :

xxx/build.sbt:25: error: not found: value assemblyMergeStrategy assemblyMergeStrategy in assembly <<= (assemblyMergeStrategy in assembly) { ^ [error] Type error in expression

The source of inspiration was this blog. Also tried the provided version as mergeStrategy might have been replaced by assemblyMergeStrategy.