Add the Spring IO plugin

The Spring IO plugin is applied to all subprojects, but only when the
platformVersion property is set. This property is used to determine
the version of the Spring IO platform that the springIoCheck task
will use. In addition to applying the plugin, the Spring Snapshots
repository is also added when the platformVersion property is set.
This allows springIoCheck to be run with snapshot versions of the
platform while leaving the build unaffected when the platformVersion
property is not set.