Try this and it should work with maven-shade-plugin version 2.1:
<transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
<manifestEntries>
<Main-Class>your.main.class</Main-Class>
<Class-Path>your/class/path/here</Class-Path>
</manifestEntries>
</transformer>
Mark the difference in tag <Class-Path>
as you had <classPath>