Add selenium to avoid loading issue in 'action_dispatch/system_test_case'

On Rails 6 we add error in test suite like:
```
LoadError: cannot load such file -- selenium/webdriver System test integration requires Rails >= 5.1 and has a hard dependency on a webserver and `capybara`, please add capybara to your Gemfile and configure a webserver (e.g. `Capybara.server = :webrick`) before attempting to use system specs.
```

Thoses errors come from
`lib/rspec/rails/example/system_example_group.rb#L51`:

```
2.6.3 :002 > require 'action_dispatch/system_test_case'
Traceback (most recent call last):
       16: from /Users/bti/code/rspec-dev/repos/rspec-rails/spec/rspec/rails/example/system_example_group_spec.rb:11:in `block (4 levels) in <module:Rails>'
       15: from /Users/bti/code/rspec-dev/repos/rspec-rails/spec/rspec/rails/example/system_example_group_spec.rb:11:in `include'
       14: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/concern.rb:122:in `append_features'
       13: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/concern.rb:122:in `class_eval'
       12: from /Users/bti/code/rspec-dev/repos/rspec-rails/lib/rspec/rails/example/system_example_group.rb:50:in `block in <module:SystemExampleGroup>'
       11: from <internal:prelude>:145:in `irb'
       10: from (irb):2:in `block in <module:SystemExampleGroup>'
        9: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:325:in `require'
        8: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:291:in `load_dependency'
        7: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:325:in `block in require'
        6: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:325:in `require'
        5: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/actionpack-6.0.0/lib/action_dispatch/system_test_case.rb:7:in `<top (required)>'
        4: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:325:in `require'
        3: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:291:in `load_dependency'
        2: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:325:in `block in require'
        1: from /Users/bti/.rvm/gems/ruby-2.6.3/gems/activesupport-6.0.0/lib/active_support/dependencies.rb:325:in `require'
LoadError (cannot load such file -- selenium/webdriver)
```

This is probably related to:
> rails/rails#36592

We need to load selenium-driver if we are in Rails 6

Author: benoittgt

Gemfile

@@ -32,6 +32,7 @@ end
 
 if MAJOR >= 6
   gem 'sqlite3', '~> 1.4', platforms: [:ruby]
+  gem 'selenium-webdriver', '~> 3.5', :require => false
 else
   gem 'sqlite3', '~> 1.3.6', platforms: [:ruby]
 end