Bump gradle version.

build.gradle

@@ -78,5 +78,5 @@ apply from: 'gradle/jar.gradle'
 apply from: 'gradle/fatjar.gradle'
 
 task wrapper(type: Wrapper) {
-    gradleVersion = '2.2.1'
+    gradleVersion = '2.4'
 }

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Thu Nov 27 00:40:06 GMT 2014
+#Fri Jun 12 22:27:48 BST 2015
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.2.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-2.4-all.zip