Upgrade Gradle Version from 2.11 to 4.1, so it's newer and consistent with the engine (which saves us 90 MiB Diskspace for Travis)

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Wed Mar 09 22:00:58 CET 2016
+#Sat Jan 13 19:07:04 CET 2018
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.11-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.1-bin.zip