Enable Style/ParenthesesAroundCondition cop

To prevent style check in review like https://github.com/rails/rails/pull/33608#discussion_r211087605.

.rubocop.yml

@@ -176,6 +176,9 @@ Lint/StringConversionInInterpolation:
 Lint/UriEscapeUnescape:
   Enabled: true
 
+Style/ParenthesesAroundCondition:
+  Enabled: true
+
 Style/RedundantReturn:
   Enabled: true
   AllowMultipleReturnValues: true

actionview/lib/action_view/helpers/text_helper.rb

@@ -228,7 +228,7 @@ def excerpt(text, phrase, options = {})
       #   pluralize(2, 'Person', locale: :de)
       #   # => 2 Personen
       def pluralize(count, singular, plural_arg = nil, plural: plural_arg, locale: I18n.locale)
-        word = if (count == 1 || count =~ /^1(\.0+)?$/)
+        word = if count == 1 || count =~ /^1(\.0+)?$/
           singular
         else
           plural || singular.pluralize(locale)

actionview/test/template/text_helper_test.rb

@@ -9,7 +9,7 @@ def setup
     super
     # This simulates the fact that instance variables are reset every time
     # a view is rendered.  The cycle helper depends on this behavior.
-    @_cycles = nil if (defined? @_cycles)
+    @_cycles = nil if defined?(@_cycles)
   end
 
   def test_concat

activerecord/lib/active_record/associations/builder/belongs_to.rb

@@ -34,7 +34,7 @@ def belongs_to_counter_cache_after_update(reflection)
           foreign_key  = reflection.foreign_key
           cache_column = reflection.counter_cache_column
 
-          if (@_after_replace_counter_called ||= false)
+          if @_after_replace_counter_called ||= false
             @_after_replace_counter_called = false
           elsif association(reflection.name).target_changed?
             if reflection.polymorphic?

guides/rails_guides/levenshtein.rb

@@ -12,8 +12,8 @@ def self.distance(str1, str2)
       n = s.length
       m = t.length
 
-      return m if (0 == n)
-      return n if (0 == m)
+      return m if 0 == n
+      return n if 0 == m
 
       d = (0..m).to_a
       x = nil

railties/lib/rails/command/spellchecker.rb

@@ -24,8 +24,8 @@ def levenshtein_distance(str1, str2) # :doc:
             n = s.length
             m = t.length
 
-            return m if (0 == n)
-            return n if (0 == m)
+            return m if 0 == n
+            return n if 0 == m
 
             d = (0..m).to_a
             x = nil