Add the Spring IO plugin to Splunk extension

Configure the Spring IO plugin such that it's only applied when the
build is run with -PplatformVersion=<version>. This platformVersion
property is used to determine the version of the Platform that will
be used when running the springIoCheck task. The plugin can be used
by running a build as follows:

./gradlew clean springIoCheck -PplatformVersion=1.1.0.BUILD-SNAPSHOT -PJDK7_HOME=… -PJDK8_HOME=…

This will test the project on JDK7 and JDK 8 using the dependencies
defined in the latest snapshot of Spring IO Platform 1.1.0.

build.gradle

@@ -6,6 +6,7 @@ buildscript {
 	}
 	dependencies {
 		classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.2.8'
+		classpath 'org.springframework.build.gradle:spring-io-plugin:0.0.3.RELEASE'
 	}
 }
 
@@ -14,6 +15,18 @@ apply from:   "${rootProject.projectDir}/publish-maven.gradle"
 apply plugin: 'eclipse'
 apply plugin: 'idea'
 
+if (project.hasProperty('platformVersion')) {
+	apply plugin: 'spring-io'
+
+	repositories {
+		maven { url "https://repo.spring.io/libs-snapshot" }
+	}
+
+	dependencies {
+		springIoVersions "io.spring.platform:platform-versions:${platformVersion}@properties"
+	}
+}
+
 group = 'org.springframework.integration'
 
 repositories {