Updated Maven Shade plug-in

There seems to be an issue with Maven 3.1+ and `maven-shade-plugin:2.0`.
When running `mvn install`, the following error is encountered:

    [ERROR] Failed to execute goal org.apache.maven.plugins:maven-shade-plugin:2.0:shade (default)
    Execution default of goal org.apache.maven.plugins:maven-shade-plugin:2.0:shade failed:
    A required class was missing while executing org.apache.maven.plugins:maven-shade-plugin:2.0:shade: org/sonatype/aether/version/VersionConstraint

Updating `maven-shade-plugin` to 2.1 fixes the build.