Bump to 2.1.0-SNAPSHOT
Replace maven-compat with maven-core and remove unused dependencies
Fix #27: Replace org.sonatype.aether with org.eclipse.aether

Maven 3.1 moved to Eclipse Aether which is incompatible with Sonatype Aether.
Plugins directly using Sonatype Aether doesn't work on 3.1+. It seems that
here's no way how to support both 3.0 and 3.1+ version. :(
Declare prerequisite on Maven 3.1 and use 3.1.0 APIs
Declare repeated versions using properties
Remove project.build.sourceEncoding from POM, it's already defined in parent
Fix indentation in POM
merge pom.xml
use 2.1.0-SNAPSHOT in pom.xml