Fix progress bar conflict

The reindex task requires some a 'progress_bar' file, but it's unsure
(depending on other configured gems) which is actually loaded and if the
variant of the gem that supplies it supports the expected API. The
solution is to add an explicit dependency on one implementation and use
their API. The ruby-progressbar gem seems to see the most activity still
and they are willing to address the differing API problem, which is
good.