Redefine uri classloader meaning

to use ```uri:classloader:/``` only makes sense if jruby is **really** loading from the ```Thread.currentThread().getContextClassLoader()``` (CCL), so in order to use it you first need to check ```runtime.getJRubyClassLoader.getParent``` to be CCL and if not supply some alternative.

this PR redefines ```uri:classloader:/``` to be ```runtime.getJRubyClassLoader.getParent``` which **always** in all possible situation will pick the same classloader, the one which loaded jruby itself.