ask maven to not package the servlet jar in the atomhopper war

I was getting these messages at startup:

INFO: validateJarFile(/path_to/apache-tomcat-7.0.39/webapps/atomhopper/WEB-INF/lib/servlet-api-2.5.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class

The spec says:

  The classloader that a container uses to load a servlet in a
  WAR... must not allow the WAR to override J2SE or Java servlet API
  classes.

The gist of it is that adding servlet classes to the war is a no-op
since the container's classloader isn't allowed to load them.  Telling
maven to make the servlet jar a "provided" scope will keep it out of
the war which makes startup nice and quiet.