* Install gems for rbx platform only when running rbx

or bundle install fails on ruby 3 because rubysl ~> 2.0 doesn't support ruby 3

This commit is based on this commit:
https://github.com/kaminari/kaminari/commit/59dbc12.

* CI against Ruby 3.0 again

3.0 without quoting is interpreted as 3, so change it to '3.0'.

* CI against Ruby 3.1