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

gradle/wrapper/gradle-wrapper.properties

-#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

gradlew

@@ -33,11 +33,11 @@ DEFAULT_JVM_OPTS=""
 # Use the maximum available, or set MAX_FD != -1 to use that value.
 MAX_FD="maximum"
 
-warn ( ) {
+warn () {
     echo "$*"
 }
 
-die ( ) {
+die () {
     echo
     echo "$*"
     echo
@@ -155,7 +155,7 @@ if $cygwin ; then
 fi
 
 # Escape application args
-save ( ) {
+save () {
     for i do printf %s\\n "$i" | sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/' \\\\/" ; done
     echo " "
 }

tests/build.gradle

@@ -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
 }