Maven plugin: fix dependency convergence

For incremental builds, we use plexus-build-api. This is not really good with maven 3.1.1, causing:

Dependency convergence error for org.codehaus.plexus:plexus-utils:1.5.8 paths to dependency are:
+-org.opendaylight.yangtools:binding-java-api-generator:0.7.0-SNAPSHOT
+-org.sonatype.plexus:plexus-build-api:0.0.7
+-org.codehaus.plexus:plexus-utils:1.5.8
and
+-org.opendaylight.yangtools:binding-java-api-generator:0.7.0-SNAPSHOT
+-org.codehaus.plexus:plexus-slf4j-logging:1.1
+-org.codehaus.plexus:plexus-container-default:1.5.4
+-org.codehaus.plexus:plexus-utils:1.4.5

[WARNING] Rule 0: org.apache.maven.plugins.enforcer.DependencyConvergence failed with message:
Failed while enforcing releasability the error(s) are [
Dependency convergence error for org.codehaus.plexus:plexus-utils:1.5.8 paths to dependency are:
+-org.opendaylight.yangtools:binding-java-api-generator:0.7.0-SNAPSHOT
+-org.sonatype.plexus:plexus-build-api:0.0.7
+-org.codehaus.plexus:plexus-utils:1.5.8
and
+-org.opendaylight.yangtools:binding-java-api-generator:0.7.0-SNAPSHOT
+-org.codehaus.plexus:plexus-slf4j-logging:1.1
+-org.codehaus.plexus:plexus-container-default:1.5.4
+-org.codehaus.plexus:plexus-utils:1.4.5

Clean this up as soon as the maven story for incremental build stabilizes (maven 3.2.2+?)