Consume versions from one or more external dependencies

Previously, the Spring IO versions were embedded in the plugin, thereby
tightly coupling the plugin to a particular version of Spring IO. This
commit updates the plugin to consume versions from one or more external
dependencies that have been added to the springioVersions configuration.
For example:

dependencies {
	springIoVersions 'io.spring.platform:platform-versions:1.0.0.BUILD-SNAPSHOT@properties'
}

Closes #31, #32