Update the plugin to a Maven 3.0.4 platform.

_Not proposed for merging yet; I intend to continue with the 3-ification by changing the goals to use annotations_

* Parent pom updated to current version from upstream.
* Maven core components pulled from that version.
* Obsolete Maven artifacts removed: maven-project, maven-toolchain.
* maven-compat added to avoid rewriting code that uses the ArtifactResolver.
* Other dependencies brought to current.
* enforcer configured to disable unwanted check from parent.
* junit up to 4.11 to get access to rules.

I picked 3.0.4 because that's my local baseline version.