Remove redundant code

As of 3b88a919 ConfigureResolutionStrategyTask is no longer used and
it should have been deleted as part of that commit. This commit
corrects that oversight.
Improve handling of unmapped dependencies

Previously, all unmapped dependencies were treated equally. With this
commit a distinction is now made between unmapped direct dependencies
and unmapped transitive dependencies. By default, the build will fail
in the event of one or more unmapped direct dependencies but unmapped
transitive dependencies have no effect on the build's outcome. This
behaviour can be overridden via the  springioPlatform extension's
failOnUnmappedDirectDependency and failOnUnmappedTransitiveDependency
properties respectively. For example, to only fail on unmapped
transitive dependencies:

springioPlatform {
	failOnUnmappedDirectDependency = false
	failOnUnmappedTransitiveDependency = true
}
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.
Update Spring IO versions