Add Ruby 3.0 to CI

[oturpe]

jekyll-feed will be included in Fedora 34 which will also use Ruby 3.0
as system Ruby interpreter. This combination is known to work, so it
would make sense to add it to CI.

[oturpe]

Both Travis and AppVeyor are in use. The reason for this is unclear for me. I updated only Travis, because it had newer versions of everything. Should also AppVeyor be updated?

[oturpe]

Oh, build for Ruby 3.0 failed due to missing rss gem. In Ruby 3.0, it became a bundled gem, thus it needs to be added as a dependency. I will fix that in this pull request. On the other hand, I see that there is some GitHub actions work going on the master branch, does that mean that Travis on its way out?

[DirtyF]

@oturpe yes the plan is to move all our CI to GitHub Actions, but we still need to test 3.9 and 4.2

[oturpe]

Rss gem fix added. Very common problem in Ruby 3.0 migration, because there was no warning in Ruby 2.7. Luckily, easy to fix too.

You can either merge this pull request, or just take the good parts if that fits the plan better.

[oturpe]

The solution for the rss problem that went is better than my proposal. Ruby 3.0 is now included in ci, so what I was trying to achieve with this pull request has been achieved. Thank you for your effort!

[ashmaroli]

Thank you @oturpe
@jekyllbot: merge +dev