MIME4J-289 Upgrade maven-assembly-plugin 3.0.0 -> 3.1.1

 - Use goals `single` instead of `attached` by the recommendation at
http://maven.apache.org/plugins-archives/maven-assembly-plugin-2.5.5/attached-mojo.html
diff --git a/assemble/pom.xml b/assemble/pom.xml
index 24a262b..cc049a0 100644
--- a/assemble/pom.xml
+++ b/assemble/pom.xml
@@ -56,6 +56,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-assembly-plugin</artifactId>
+                <version>3.1.1</version>
                 <configuration>
                     <descriptorSourceDirectory>${basedir}/src/assemble/</descriptorSourceDirectory>
                     <tarLongFileMode>gnu</tarLongFileMode>
@@ -65,7 +66,7 @@
                         <id>make-assembly</id>
                         <phase>package</phase>
                         <goals>
-                            <goal>attached</goal>
+                            <goal>single</goal>
                         </goals>
                     </execution>
                 </executions>