[bugfix] Do not expose raw type in API

Raw types should not be exposed through an API as this makes integration with some non-Java JVM languages impossible, e.g. Scala.

It was previously impossible to implement hudson.tasks.BuildStepDescriptor::isApplicable in Scala due to a raw type.

For background see - http://stackoverflow.com/questions/5279149/implementing-methods-having-raw-types-in-scala/5279343#5279343