Uniform jruby home

the jruby home detection is an interesting thing - but there is well defined way of just setting it: just use the one from the classloader.

this patch would make much more sense if the "detection" of the parent classloader of the runtime.jrubyClassLoader works reliable: https://github.com/jruby/jruby/pull/2023
and if the uri:classloader: protocol will use this parent classloader for the lookup: https://github.com/jruby/jruby/pull/2027

even without the other two PR it reduces the setting of  jruby.home to two use-case:
* jruby home is on the file system and bin/jruby sets it via the property
* jruby home lives inside a jar file: uri:classloader:/META-INF/jruby.home

the first case is already well tested. the second is not tested at all in the sense that jruby-complete currently hardly runs any tests. so I added the test/jruby.index using the jruby-complete to run them:
```
mvn -Pjars-test
```
and allow failures there for travis since it does not pass. well even without the jruby patches applied it does not pass.

well, the whole things fails when tools like "jruby-rack" to detect jruby home itself and overwrite the setting coming from jruby itself.

@enebo @ratnikov