RDF serializations for individual places failing

[paregorios]

URIs like https://pleiades.stoa.org/places/939242064/turtle are throwing tracebacks like:

Traceback (innermost last):

    Module ZPublisher.Publish, line 138, in publish
    Module ZPublisher.mapply, line 77, in mapply
    Module ZPublisher.Publish, line 48, in call_object
    Module pleiades.rdf.browser, line 35, in __call__
    Module pleiades.rdf.browser, line 24, in graph
    Module pleiades.rdf.common, line 353, in place
    Module pleiades.rdf.common, line 181, in public_url

TypeError: urljoin() takes at least 2 arguments (1 given) 

Looks like the weekly catalog export dumps to RDF are fine.

[jessesnyder]

@paregorios I think I introduced this bug (surprise, surprise) with the recent export work. I'm pretty confident about a fix, but I won't be able to stage or deploy due to my IP address instability.

I've made what is hopefully a fix with a PR to pleiades-rdf, and merged the change to the staging branch on that repository.