Use ChildFirstClassLoader from http://articles.qos.ch/classloader.html.

JCL does not properly implement the ClassLoader.getResource function,
and thus does not work for embedding JRuby. I've submitted a pull
request for partially implementing this support, but I don't know if
it will be accepted upstream.

https://github.com/kamranzafar/JCL/pull/10

If JCL does eventually accept that pull request we can switch back to
using it instead of this bare bones class loader.
Fix class loading issue due to URLClassLoader's parent-first policy.