Upgrade gradle.

Needed for java 9 support.

build.gradle

@@ -12,6 +12,6 @@ tasks.dependencies.dependsOn subprojects*.tasks.dependencies
 
 task wrapper(type: Wrapper) {
     description 'Downloads and installs an up-to-date wrapper for gradle.'
-    gradleVersion = '3.1'
+    gradleVersion = '4.6'
 }
 

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Thu Oct 27 18:01:10 BST 2016
+#Sat Mar 31 12:30:32 BST 2018
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.6-bin.zip