upgrade Gradle wrapper

build.gradle

@@ -73,7 +73,7 @@ subprojects {
 }
 
 task wrapper(type: Wrapper) {
-    gradleVersion = '3.4.1'
+    gradleVersion = '4.0'
 }
 
 //Task used by Heroku for staging

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Fri Mar 17 10:32:10 CET 2017
+#Fri Jun 23 10:27:01 CEST 2017
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.4.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.0-bin.zip