Drop support for Ruby 2.3.

This commit drops support for Ruby 2.3 and suppresses the following
`Gemspec/RequiredRubyVersion` offense.

```console
% cd path/to/rubocop-rake
% bundle exec rake
(snip)

Offenses:

rubocop-rake.gemspec:14:32: C: Gemspec/RequiredRubyVersion:
required_ruby_version (2.3, declared in rubocop-rake.gemspec) and
TargetRubyVersion (2.4, which may be specified in .rubocop.yml) should
be equal.
  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

25 files inspected, 1 offense detected
```

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Changes
+
+* [#33](https://github.com/rubocop-hq/rubocop-rake/pull/33): Drop support for Ruby 2.3. ([@koic][])
+
 ## 0.5.1 (2020-02-14)
 
 ### Bug fixes
@@ -51,3 +55,4 @@
 
 [@pocke]: https://github.com/pocke
 [@jaruuuu]: https://github.com/jaruuuu
+[@koic]: https://github.com/koic

rubocop-rake.gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{A RuboCop plugin for Rake}
   spec.description   = %q{A RuboCop plugin for Rake}
   spec.homepage      = "https://github.com/rubocop-hq/rubocop-rake"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
   spec.licenses = ['MIT']
 
   spec.metadata["allowed_push_host"] = "https://rubygems.org"