Bumping to gradle 4.1. (#2399)

Refer to https://docs.gradle.org/current/release-notes.html for the full release notes.

- Zinc updated (incremental Scala compiler)
- Better logging for parallel tasks (useful locally when working with redo)
- Build cache improvements

Author: markusthoemmes

gradle/wrapper/gradle-wrapper.jar

@@ -1,6 +1,5 @@
-#Tue Apr 25 07:35:34 CEST 2017
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.5-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.1-bin.zip

gradle/wrapper/gradle-wrapper.properties

@@ -61,7 +61,7 @@ tasks.withType(ScalaCompile) {
     scalaCompileOptions.additionalParameters = gradle.scala.compileFlags
 }
 
-def keystorePath = new File(buildDir, 'classes/test/keystore')
+def keystorePath = new File(sourceSets.test.scala.outputDir, 'keystore')
 task deleteKeystore(type: Delete) {
     delete keystorePath
 }