Fix for tomcat 6.0.35 (ubuntu 11, 12)

The problem for tomcat 6.0.35 in the current petclinic source was that static content could not be found (http://grokbase.com/t/tomcat/users/1265vgfngm/help-linking-static-content-in-a-webapp-before-and-after-6-0-30-x). With my servlet-mapping it's gone.