Update RSpec/Dialect config

so that all Capybara-specific methods that have the same native RSpec method
(e.g. are just aliases) following the upgrade to rubocop-rspec 3.0.

See:
- https://github.com/rubocop/rubocop-rspec/pull/1848/files
- rubocop/rubocop-rspec@1fa9ce3

> If you were previously using the ‘RSpec/Capybara/FeatureMethods` cop and want to keep disabling all Capybara-specific methods that have the same native RSpec method (e.g. are just aliases), use the following config:
>
> ```
> RSpec/Dialect:
>    PreferredMethods:
>      background: :before
>      scenario:   :it
>      xscenario:  :xit
>      given:      :let
>      given!:     :let!
>      feature:    :describe
> ```

config/rspec.yml

@@ -56,6 +56,15 @@ RSpec/ContextWording:
 # scenario.
 # We don't want this cop outside of feature or system specs though.
 RSpec/Dialect:
+  # Disables all Capybara-specific methods that have the same native 
+  # RSpec method (e.g. are just aliases)
+  PreferredMethods:
+    background: :before
+    scenario:   :it
+    xscenario:  :xit
+    given:      :let
+    given!:     :let!
+    feature:    :describe
   Exclude:
     - 'spec/features/**/*.rb'
     - 'spec/system/**/*.rb'