Update Gemfile.lock
Keep history of processed (seen) entries

Major rewrite to replace previous behavior where each entry's pubdate
was compared to the last fetch datetime of its feed to determine whether
it had been processed before or not. This approach has several
implications since many feeds have entries with invalid, missing or even
future pubdates (previous behavior was to ignore the entry until its
pubdate passed). It also introduces a condition where some entries may
not be processed at all.

For example, suppose feed2email runs every hour. A website creates a new
entry at 11:50 (its pubdate) and makes it available online (in its feed)
at 12:10, causing feed2email to miss the entry when run at 12:00. Next
time it runs (13:00), it will compare the entry's pubdate (11:50) with
the feed's last fetch datetime (12:00) and think it has already
processed it, thus skipping it!
Handle feed fetching/parsing errors
Bump to version 0.4.0