Drop support for Ruby < 2.4

With Ruby 2.4 being EOL, we should drop support for old versions to
reduce Rouge gem maintenance effort. It also helps to reduce build
runtime and avoid overrunning our month CI build time allotment.

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.0
+  TargetRubyVersion: 2.5
   DisplayCopNames: true
   Exclude:
     - "tasks/**/*"

.travis.yml

@@ -1,11 +1,6 @@
 dist: trusty
 language: ruby
 rvm:
-  - "2.0"
-  - "2.1"
-  - "2.2"
-  - "2.3"
-  - "2.4"
   - "2.5"
   - "2.6"
   - &latest_ruby "2.7"

rouge.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |s|
   s.files = Dir['Gemfile', 'LICENSE', 'rouge.gemspec', 'lib/**/*.rb', 'lib/**/*.yml', 'bin/rougify', 'lib/rouge/demos/*']
   s.executables = %w(rougify)
   s.licenses = ['MIT', 'BSD-2-Clause']
-  s.required_ruby_version = '>= 2.0'
+  s.required_ruby_version = '>= 2.5'
   s.metadata = {
     "bug_tracker_uri"   => "https://github.com/rouge-ruby/rouge/issues",
     "changelog_uri"     => "https://github.com/rouge-ruby/rouge/blob/master/CHANGELOG.md",