Set npm version for frontend-maven-plugin

When using NodeJS 4+, the frontend-maven-plugin uses the bundled npm,
but it ends up not being available via PATH lookup. That causes issue
when one package.json has a script that invokes npm (it can not be
found).

Set <npmVersion> to the version being used on Wikimedia CI which would
hopefully install npm in a directory belonging to PATH.

eirslett/frontend-maven-plugin#560
https://github.com/eirslett/frontend-maven-plugin#installing-node-and-npm

Bug: T188285
Change-Id: Ia892dcfdc1c0ef055f00a493941e974198323012

pom.xml

@@ -758,6 +758,15 @@
                     <groupId>com.github.eirslett</groupId>
                     <artifactId>frontend-maven-plugin</artifactId>
                     <version>1.6</version>
+                    <configuration>
+                        <!--
+                             npm installed from NodeJS distribution turns out
+                             to not be in PATH which has side effects.
+                             https://phabricator.wikimedia.org/T188285
+                             https://github.com/eirslett/frontend-maven-plugin/issues/560
+                        -->
+                        <npmVersion>3.8.3</npmVersion>
+                    </configuration>
                 </plugin>
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>