Update sorl to get rid of deprecation warning

sorl 11.12 uses django.util.simplejson, which is deprecated.  That dependency has been removed in sorl 11.12.1b.  It's a minor version bump, so I don't expect any issues.  We only use sorl for image thumbnails on multimedia upload.

@dannyroberts can you give your thoughts on this?  It looks like a "hutch"-specific thing, which seems to be dmyung only.  Any idea how I can test this, exactly?