Updated the maven shade plugin version to 2.1

The maven shade plugin breaks the build with following error 

java.lang.NoClassDefFoundError: org/sonatype/aether/version/VersionConstraint
[WARNING] Error injecting: org.apache.maven.shared.dependency.graph.internal.Maven3DependencyGraphBuilder
The error details can be found at
https://cwiki.apache.org/confluence/display/MAVEN/AetherClassNotFound

This error is caused by Maven 3.1-alpha-1 migration from Sonatype Aether to Eclipse Aether (MNG-5354), which is an incompatible change for some plugins.
The fix is to update plugin version to 2.1