`rss` gem is moved to bundled gem

c.f. https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/

(cherry picked from commit 53aac0d)

```
  1) Error:
LargeGemListSpec#test_method: more than 5 versions of the same gem :
ArgumentError: wrong number of arguments (given 2, expected 0..1)
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/rack_test/node.rb:67:in `click'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/rack_test/node.rb:131:in `call'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/rack_test/node.rb:131:in `click'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-mechanize-1.11.0/lib/capybara/mechanize/node.rb:12:in `click'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/node/element.rb:172:in `block in click'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/node/element.rb:597:in `block in perform_click_action'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/node/base.rb:83:in `synchronize'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/node/element.rb:597:in `perform_click_action'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/node/element.rb:171:in `click'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/node/actions.rb:42:in `click_link'
    /Users/sue445/workspace/github.com/geminabox/geminabox/vendor/bundle/ruby/3.0.0/gems/capybara-3.35.3/lib/capybara/session.rb:762:in `click_link'
    /Users/sue445/workspace/github.com/geminabox/geminabox/test/integration/web_interface/large_gem_list_test.rb:32:in `block in <class:LargeGemListSpec>'

```

c.f. phillbaker/capybara-mechanize#68

(cherry picked from commit 4fa2f7a)

Latest master branch (will be next version) of capybara-mechanize is requires ruby 2.3.0+
phillbaker/capybara-mechanize@ec29688#diff-169a230d5998eabe69cc2134c7dbf5ac54f45bfce1c4752cf48ed96b73466526R9

c.f. https://travis-ci.org/github/geminabox/geminabox/jobs/761589443

(cherry picked from commit 76eb79e)

rubygems 2.7.9 doesn't work on Ruby 3.0. So I use latest rubygems on Ruby 3.0

https://travis-ci.org/github/geminabox/geminabox/jobs/764702976

```
Resolving dependencies...
Using rake 13.0.3
Fetching public_suffix 4.0.6
/home/travis/.rvm/rubies/ruby-3.0.0/lib/ruby/site_ruby/3.0.0/bundler/shared_helpers.rb:35: warning: Pathname#untaint is deprecated and will be removed in Ruby 3.2.
Bundler::GemspecError: Could not read gem at
/home/travis/.rvm/gems/ruby-3.0.0/cache/public_suffix-4.0.6.gem. It may be
corrupted.
An error occurred while installing public_suffix (4.0.6), and
Bundler cannot continue.
Make sure that `gem install public_suffix -v '4.0.6' --source
'https://rubygems.org/'` succeeds before bundling.
In Gemfile:
  capybara-mechanize was resolved to 1.11.0, which depends on
    capybara was resolved to 3.35.3, which depends on
      addressable was resolved to 2.7.0, which depends on
        public_suffix
The command "eval bundle install --without development " failed 3 times.

```

```
[DEPRECATED] `Bundler.with_clean_env` has been deprecated in favor of `Bundler.with_unbundled_env`. If you instead want the environment before bundler was originally loaded, use `Bundler.with_original_env` (called at /Users/sue445/workspace/github.com/geminabox/geminabox/test/test_support/geminabox_test_case.rb:123)
```

c.f. https://travis-ci.org/github/geminabox/geminabox/jobs/764726229#L502

Fixed. doesn't work on Ruby 3.0