Gradle, js, karma: use ^4.0.0-rc.6 to use webpack 3, require webpack

#KT-31564 Fixed

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/NpmVersions.kt

@@ -35,7 +35,7 @@ class NpmVersions {
     val karmaSafariLauncher = NpmPackageVersion("karma-safari-launcher", "1.0.0")
 
     val karmaMocha = NpmPackageVersion("karma-mocha", "1.3.0")
-    val karmaWebpack = NpmPackageVersion("karma-webpack", "3.0.5")
+    val karmaWebpack = NpmPackageVersion("karma-webpack", "^4.0.0-rc.6")
     val karmaCoverage = NpmPackageVersion("karma-coverage", "1.1.2")
 
     val karmaSourceMapLoader = NpmPackageVersion("karma-sourcemap-loader", "0.3.7")

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/testing/karma/KotlinKarma.kt

@@ -104,6 +104,7 @@ class KotlinKarma(override val compilation: KotlinJsCompilation) : KotlinJsTestF
 
     private fun useWebpack() {
         requiredDependencies.add(versions.karmaWebpack)
+        requiredDependencies.add(versions.webpack)
 
         addPreprocessor("webpack")
         confJsWriters.add {