Improvements to handling reindexing of docs by removing _version_ field if present and handling metrics with slash

[justinrsweeney]

In some cases we want to use solr-bench to reindex docs that are exported from a Solr instance and may include the _version_ field. This cannot be included in a doc to index so added code to remove it if present.

Additionally this includes some housekeeping items to:

1. Upgrade the jarchivelib to avoid logging lots of warnings on newer versions of Java
2. Add to readme to allow for using python3 if installed to view results

[chatman]

Thanks @justinrsweeney