Only re-download if the server file is newer

That way you can run it again and easily get updates only. 
Also if you get to the ogr2ogr command and discover it hasn't 
been installed and you need to rerun it, get-shapefiles.sh 
won't redownload all the data again.
Merge pull request #31 from smsm1/patch-1

Only re-download if the server file is newer