Remove testing for Ruby versions 2.4 and older (#174)

.github/workflows/ci.yml

@@ -29,7 +29,6 @@ jobs:
           - "2.7"
           - "2.6"
           - "2.5"
-          - "2.4"
           - "jruby"
         include:
           - ruby: "2.6"

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.4
+  TargetRubyVersion: 2.5
   Exclude:
     - spec/support/coverage.rb
     - spec/support/warnings.rb

.travis.yml

@@ -1,28 +0,0 @@
-language: ruby
-cache: bundler
-bundler_args: --without tools benchmarks
-before_install:
-  - gem update --system
-  - gem install bundler
-script:
-  - bundle exec rake
-before_script:
-  - curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter
-  - chmod +x ./cc-test-reporter
-  - ./cc-test-reporter before-build
-after_script:
-  - "[ -d coverage ] && ./cc-test-reporter after-build --exit-code $TRAVIS_TEST_RESULT"
-rvm:
-  - 2.6.3
-  - 2.5.5
-  - 2.4.6
-  - 2.3.8
-  - jruby-9.2.7.0
-notifications:
-  email: false
-  webhooks:
-    urls:
-      - https://webhooks.gitter.im/e/19098b4253a72c9796db
-    on_success: change # options: [always|never|change] default: always
-    on_failure: always # options: [always|never|change] default: always
-    on_start: false # default: false

Gemfile.devtools

@@ -10,7 +10,7 @@ group :test do
 
   gem "codacy-coverage", require: false, platforms: :ruby
 
-  gem "warning" if RUBY_VERSION >= "2.4.0"
+  gem "warning"
 end
 
 group :tools do