eliminate language warnings

Author: SethTisue

build.sbt

@@ -5,7 +5,8 @@ inThisBuild(Def.settings(
   version := "0.1-SNAPSHOT",
   resolvers += "scala-pr" at "https://scala-ci.typesafe.com/artifactory/scala-integration/",
   crossScalaVersions := Seq("2.12.5", "2.13.0-pre-b11db01", "2.11.12"),
-  scalaVersion := crossScalaVersions.value.head
+  scalaVersion := crossScalaVersions.value.head,
+  scalacOptions ++= Seq("-feature", "-language:higherKinds", "-language:implicitConversions")
 ))
 
 lazy val `scala-collection-compat` = crossProject(JSPlatform, JVMPlatform)