Update Gradle wrapper to 3.1

Author: friederbluemle

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Tue Aug 02 23:08:04 SGT 2016
+#Tue Sep 27 18:02:33 SGT 2016
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.14.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-3.1-all.zip