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.