Upgrade dom4j to avoid warning related to Java 9

Building the project created by the multi-module archetype results in this warning:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.dom4j.io.SAXContentHandler (file:/Users/basilbourque/.m2/repository/dom4j/dom4j/1.6.1/dom4j-1.6.1.jar) to method com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser$LocatorProxy.getEncoding()
WARNING: Please consider reporting this to the maintainers of org.dom4j.io.SAXContentHandler
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

Some googling leads me to believe this relates to a Java 9 problem in earlier versions of the dom4j project. According to this Issue # 499:

https://github.com/spotbugs/spotbugs/issues/499

…version 2.1.0 of dom4j max resolve the issue.

⇒ Consider upgrading the spotbugs library from one using dom4j 1.6.1 to one using dom4j 2.1.0 or later.

My environment: Vaadin Framework 8.2.0, Oracle JDK for macOS version 9.0.1, macOS Sierra, IntelliJ 2017.3.

Start by locating the dependency used by the multi-module archetype that brings in dom4j 1.6.1, then review how the SpotBugs library is declared. Update it to a version using dom4j 2.1.0 or later and build the generated multi-module project on Java 9 to confirm the illegal reflective access warning is gone.