create the jruby spawner with the URL of runtime.jruby_class_loader.parent.urls

this allows run the tests from various setups: jruby-complete, jruby-core+jruby-stdlib
or the module jruby maven artifact with all its dependencies. actually the surefire launcher
combines all those jars into one temporary jar for launching the tests. in this manner
it is enough to pick only two jruby maven aritfact and jruby-jars and with this jruby-complete
is already covered.

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