Fix method that fails in Christmas
Fix method that fails in Christmas

The `birthdays_of_this_week` method worked always except when executing
it from 26 to 31 December as we hadn't have into account the new year
break.

Fixes https://github.com/openSUSE/agile-team-dashboard/issues/98.

freakuency/agile-team-dashboard,freakuency/agile-team-dashboard,freakuency/agile-team-dashboard,freakuency/agile-team-dashboard