Upgrade to Gradle 2.x

The Sonar server, sonar.spring.io, was recently upgraded and is no
longer compatible with the Sonar client that's used by Gradle 1.x.
This commit upgrades the build to Gradle 2.x; restoring compatibility
with sonar.spring.io.

See gh-9

build.gradle

@@ -126,5 +126,5 @@ artifacts {
 }
 
 task wrapper(type: Wrapper) {
-	gradleVersion = '1.11'
+	gradleVersion = '2.2.1'
 }

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Mon Mar 24 13:51:24 GMT 2014
+#Tue Dec 02 09:48:49 GMT 2014
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=http\://services.gradle.org/distributions/gradle-1.11-bin.zip
+distributionUrl=http\://services.gradle.org/distributions/gradle-2.2.1-bin.zip

user-guide/build.gradle

@@ -3,7 +3,7 @@ buildscript {
         maven { url 'http://repo.spring.io/plugins-release' }
     }
     dependencies {
-        classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.2.1'
+        classpath 'io.spring.gradle:docbook-reference-plugin:0.3.0'
     }
 }