Loosen up ruby-progressbar version constraint

Leaving off the `PATCH` level of the version constraint will make this gem compatible with future versions of ruby-progressbar up until the next major version. This is to lessen the need for manual updates, and increase the chances of staying compatible with e.g. fuubar.

Seeing as @jfelchner stated his intent to use semantic versioning for ruby-progressbar in [a comment on #21][21], we should be safe. :)

  [21]: https://github.com/martinciu/fuubar-cucumber/pull/21#issuecomment-7500330