ignore the jar soup
libs now reside under spo/lib

`spo/lib` is automatically included by solr, since

> all directories and paths are resolved relative to the `instanceDir`.

For now it's the task of the user to add the libraries
per hand, depending on the hosts' setup. (A rant under
http://comments.gmane.org/gmane.comp.jakarta.lucene.solr.user/75775
seems to underline, that "Solr is a second-class citizen when it comes
to production deployment.")