Change URLs for remote data to avoid jefferislab.org redirect

``download.file`` periodically decides it does not want to work with the redirect set up from ``jefferislab.org`` to ``flybrain.mrc-lmb.cam.ac.uk``, as used in ``read.neuronlist``, etc. in [nat](https://github.com/jefferis/nat). Until this issue is solved, this works around it by changing the URLs to the redirect target, namely ``flybrain.mrc-lmb.cam.ac.uk``.