Drop support for Ruby 2.3

RuboCop has dropped support for Ruby 2.3
(rubocop/rubocop@5873894)
and so should we.

.circleci/config.yml

@@ -21,16 +21,6 @@ jobs:
       - run: bundle install
       - run: rake confirm_config documentation_syntax_check confirm_documentation
 
-  # Ruby 2.3
-  ruby-2.3-rspec:
-    docker:
-      - image: circleci/ruby:2.3
-    <<: *rspec
-  ruby-2.3-rubocop:
-    docker:
-      - image: circleci/ruby:2.3
-    <<: *rubocop
-
   # Ruby 2.4
   ruby-2.4-rspec:
     docker:
@@ -128,10 +118,6 @@ workflows:
       - confirm_config_and_documentation
 
       # Use `requires: [confirm_config_and_documentation]` to trick Circle CI into starting the slow jruby job early.
-      - ruby-2.3-rspec:
-          requires: [confirm_config_and_documentation]
-      - ruby-2.3-rubocop:
-          requires: [confirm_config_and_documentation]
       - ruby-2.4-rspec:
           requires: [confirm_config_and_documentation]
       - ruby-2.4-rubocop:

.rubocop.yml

@@ -5,7 +5,7 @@ require:
 
 AllCops:
   DisplayCopNames: true
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.4
   Exclude:
     - 'vendor/**/*'
     - 'spec/fixtures/**/*'

rubocop-rspec.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |spec|
 
   spec.version = RuboCop::RSpec::Version::STRING
   spec.platform = Gem::Platform::RUBY
-  spec.required_ruby_version = '>= 2.3.0'
+  spec.required_ruby_version = '>= 2.4.0'
 
   spec.require_paths = ['lib']
   spec.files = Dir[