feed: use hashlib.sha1 for fallback IDs in Feed._get_entry_id

feed: use hashlib.sha1 for fallback IDs in Feed._get_entry_id

This fixes what I broke in

  commit 0fc2ff4465d741823b3dceebcfcf3a98a0081522
  Author: W. Trevor King <wking@tremily.us>
  Date:   Thu Oct 4 08:33:32 2012 -0400

    Cleanup global module configuration.

  diff --git a/rss2email.py b/rss2email.py
  index 216c13d..3919bd7 100755
  --- a/rss2email.py
  +++ b/rss2email.py
  ...
  -hash = hashlib.md5
  ...

I've come back with sha1 instead of md5, mostly because that's what
Git uses ;).  We don't migrate recorded IDs from the old configuration
method, so the change from md5 to sha1 shouldn't affect anyone.

Signed-off-by: W. Trevor King <wking@tremily.us>