Allow to pass empty md5sum.

slapos/recipe/build.py

@@ -163,7 +163,7 @@ class Script:
       raise zc.buildout.UserError('Promise not met, found issues:\n  %s' %
           '  '.join([q + '\n' for q in promise_problem_list]))
 
-  def download(self, url, md5sum):
+  def download(self, url, md5sum=None):
     download = zc.buildout.download.Download(self.buildout['buildout'],
         hash_name=True, cache=self.buildout['buildout'].get('download-cache'))
     path, is_temp = download(url, md5sum=md5sum)