Merge pull request #643 from exercism/gradle-update

Update Gradle wrapper to 4.0

gradle/wrapper/gradle-wrapper.properties

-#Tue May 23 10:48:11 BST 2017
+#Wed Jun 14 14:21:29 EDT 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-4.0-bin.zip