Enable RuboCop

- RuboCop version is locked to 0.81.0 which supports Ruby 2.3
Once Ransack bumps the required_ruby_version, RuboCop can be bumped.

- `DisabledByDefault: true` not to apply all of default cops

- Some Layout and Style cops are enabled to make initial changes small
Other cops can be enabled later.

.rubocop.yml

@@ -0,0 +1,42 @@
+AllCops:
+  # TargetRubyVersion value is based on `required_ruby_version = '>= 2.3'` in the ransack.gemspec
+  TargetRubyVersion: 2.3
+
+  DisabledByDefault: true
+
+Layout/EmptyLineAfterMagicComment:
+  Enabled: true
+
+Layout/EmptyLineBetweenDefs:
+  Enabled: true
+
+Layout/EmptyLines:
+  Enabled: true
+
+Layout/FirstArrayElementIndentation:
+  EnforcedStyle: consistent
+
+Layout/SpaceAfterComma:
+  Enabled: true
+
+Layout/SpaceInsideBlockBraces:
+  Enabled: true
+
+Layout/SpaceInsideHashLiteralBraces:
+  Enabled: true
+
+Layout/SpaceInsideParens:
+  Enabled: true
+
+Layout/TrailingEmptyLines:
+  Enabled: true
+
+Style/RedundantReturn:
+  Enabled: true
+
+Style/SelfAssignment:
+  Enabled: true
+
+Style/Semicolon:
+  Enabled: true
+

Gemfile

@@ -49,3 +49,8 @@ group :test do
   gem 'rspec', '~> 3'
   gem 'simplecov', :require => false
 end
+
+# RuboCop 0.81.0 is the last version which supports Ruby 2.3.
+# Once Ransack required_ruby_version is bumped, RuboCop version can be bumped.
+# https://github.com/rubocop-hq/rubocop/pull/7869
+gem 'rubocop', '=0.81.0', require: false