It looks like ElementTraversal is part of xml-apis-2.10.0.jar which should have been provided with your Shibboleth installation.
So if you were following these instructions you should also have followed this step:
Endorse Xerces and Xalan by creating the directory JETTY_HOME/lib/endorsed/ and copy the .jar files included in the IdP source endorsed/ directory into the newly created directory.
So long as you copied all the jar files appropriately, it should be okay. My guess is that you missed out that step – or perhaps only copied the jar files with Xalan or Xerces in the name, and missed the other two.