fix(ci): make npm version implicit

Author: duncdrum

pom.xml

@@ -50,6 +50,7 @@
 
     <exist.version>4.6.1</exist.version>
     <node.version>v11.6.0</node.version>
+    <npm.version>6.5.0</npm.version>
 
     <package-name>http://exist-db.org/apps/doc</package-name>
   </properties>
@@ -170,6 +171,7 @@
         </executions>
         <configuration>
           <nodeVersion>${node.version}</nodeVersion>
+          <npmVersion>${npm.version}</npmVersion>
         </configuration>
       </plugin>
       <plugin>