Update plugins to current versions.

maven-gpg-plugin ......................................... 1.4 -> 1.5
maven-surefire-plugin .................................. 2.16 -> 2.17
org.sonatype.plugins:nexus-staging-maven-plugin ............... 1.6.2

Minimum Maven version has been changed from 3.0 to 3.0.4 because it
is needed by the nexus-staging-maven-plugin.
Update oss-parent from version 7 to 9 (current).
Format POM with `mvn tidy:pom`.

Use canonical ordering.
Use nexus plugin with autoReleaseAfterClose.

Avoid manual step for releasing System Rules.