Add assembly plugin again.

Otherwise, the docs build will fail.

Signed-off-by: Gerrit Meier <meistermeier@gmail.com>

Author: meistermeier

pom.xml

@@ -553,6 +553,10 @@
 			</plugins>
 		</pluginManagement>
 		<plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+            </plugin>
 			<plugin>
 				<groupId>com.github.ekryd.sortpom</groupId>
 				<artifactId>sortpom-maven-plugin</artifactId>