target Maven 3.1

Breaking change that is not compatible with Maven < 3.1 .

spoon-maven-plugin: use org.eclipse.aether instead of org.sonatype.aether .
see http://maven.apache.org/docs/3.1.0/release-notes.html

spoon: use android-maven-plugin 3.8.0 which is the lowest version compatible
with Maven 3.1 .