Update Spring IO plugin configuration

- Upgrade to 0.0.3.RELEASE of the plugin
- Declare a dependency in the springIoVersions configuration on the
  Platform’s platform-versions artifact. An external property is used to
  provide the version of this artifact. This means that the build script
  does not need to refer to a snapshot artifact (the only version
  available at the time of writing) and also breaks a circular
  dependency between Spring Security (a project that’s part of the
  Platform) and the Platform itself.
- The plugin is applied and the libs-snapshot repository is added only
  if the external property is specified. This removes the possibility
  of a release or milestone being shipped with a snapshot dependency
  (unless the release build is performed with -PplatformVersion=foo).