Add the Spring IO plugin

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=1.0.0.BUILD-SNAPSHOT -PJDK8_HOME=…`
```
This will test the project on JDK 8 using the dependencies defined in the latest snapshot of Spring IO Platform 1.0.0.