Fix Papers.app bibtex output

Papers.app outputs a URL attribute for each article in its BibTeX export.  This
breaks latexmk.  There are two ways to fix this:

1. \usepackage{url}.  This helps latexmk understand the field, but then every
entry in the bibliography will have a URL field which, for the type of articles
I use Papers for, is not needed.

2. Remove the URL field from the *.bib before processing (which is what I have
done).