Make classloader handling more compatible with JDK 9

[cushon]

From http://www.oracle.com/technetwork/java/javase/9-relnote-issues-3704069.html:

The application class loader is no longer an instance of java.net.URLClassLoader (an implementation detail that was never specified in previous releases). Code that assumes that ClassLoader::getSytemClassLoader returns a URLClassLoader object will need to be updated. Note that Java SE and the JDK do not provide an API for applications or libraries to dynamically augment the class path at run-time.