MTOMCAT-234 Classloading issues

this fixes the `java.lang.ClassFormatError` but there is another problem: it seems that the `JarScanner` scans the tomcat-maven-plugin dependencies instead of the webapp ones. The result is that no `web-fragment.xml` or `ServletContainerInitializer` works anymore (and I suspect there should be other consequences as well). I did not have enough time to investigate but I noticed that, at line 202 in `StandardJarScanner`, a classloader is skipped (it wasn't so in Tomcat7)... but I don't know if this could be the cause as I don't know the internals of Tomcat very well. I'll try to make some time next week to dig a bit deeper.