Fix an issue where season banners don't always download for the most recent season

Fix an issue where season banners don't download for the most recent season of a show that does not have a season 0 (specials season).

Currently:
If show X has three seasons and specials, num_seasons is 4 (tvdb_show_obj: [0, 1, 2, 3])
If show Y has three seasons and no specials, num_seasons is 3 (tvdb_show_obj: [1, 2, 3])

When iterating over range(num_seasons) to look for season banners:
Show X will check for banners for seasons [0, 1, 2, 3]
Show Y will check for banners for seasons [0, 1, 2]

This means Sick-Beard will never download season banners for the most recent season of a series without specials.

With this patch the iteration takes place over range(num_seasons+1) so:
Show X will check for banners for seasons [0, 1, 2, 3, 4]
Show Y will check for banners for seasons [0, 1, 2, 3]