Setting the download directory so you only get one instance downloaded

[awead]

This was reported in samvera/active_fedora#1080 but was decided in samvera/active_fedora#1081 that the fix should be here in solr_wrapper.

[cbeer]

With solr_wrapper 0.14, here's what I have now:

$ cat ~/.solr_wrapper.yml 
download_dir: /Users/cabeer/tmp