Update maven-source-plugin to 2.1 and add default entries to manifest

<plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-source-plugin</artifactId>
        <configuration>
          <archive>
            <manifest>
              <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
              <addDefaultSpecificationEntries>true</addDefaultSpecificationEntries>
            </manifest>
          </archive>
        </configuration>
</plugin>