Add task to ease checking of version mappings

Previously, to check that all of a project's dependencies had Spring IO version mappings it was necessary to run the `springIoCheck` tasks, specifying a JDK 7 and/or JDK 8 home, or to run the `dependencies` task on each module.

To make this process easier, this commit adds a task, that the `springioCheck` task depends on, which causes the `springioTestRuntime` configuration to be resolved thereby triggering a check of its dependency version mapping.

To check that a project's configuration is in good shape you can run `./gradlew springioCheck --continue`. This will check for incomplete excludes, dependencies with preferred alternatives, and for dependencies that don't have an approved Spring IO version.

This PR should be applied after #28