Drop support for Ruby 2.3 (#942)

Signed-off-by: Rémy Coutable <remy@rymai.me>

.rubocop.yml

@@ -4,7 +4,7 @@ inherit_from:
 
 # Files you want to exclude
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.4
   Exclude:
     - vendor/**/*
     - db/schema.rb

.travis.yml

@@ -1,6 +1,5 @@
 language: ruby
 rvm:
-  - 2.3.8
   - 2.4.9
   - 2.5.7
   - 2.6.5
@@ -14,12 +13,14 @@ jdk:
 env:
   global:
     - JRUBY_OPTS="--debug"
-jdk:
-  - openjdk8
+
 script:
   - bundle exec rake
+
 bundler_args: --without development
+
 cache: bundler
+
 addons:
   code_climate:
     repo_token: 185a714e42214b9369217d0ddfd5a02d528d9a616a34482119a16c63241d6afd

README.md

@@ -1,6 +1,10 @@
 # Guard
 
-**IMPORTANT: [Ruby 2.1 is officially outdated and unsupported!](https://www.ruby-lang.org/en/news/2016/03/30/ruby-2-1-9-released/) Please upgrade to Ruby >=2.3 or >=2.2.9 before installing Guard! To install for older versions, update Bundler at least 1.12: `gem update bundler` and Bundler should correctly resolve to earlier gems for your given Ruby version.**
+**IMPORTANT: Please upgrade to Ruby >= 2.4 before installing Guard! To install for older versions, update Bundler at least 1.12: `gem update bundler` and Bundler should correctly resolve to earlier gems for your given Ruby version.**
+
+- [Ruby 2.1 is officially outdated and unsupported!](https://www.ruby-lang.org/en/news/2016/03/30/ruby-2-1-9-released/)
+- [Ruby 2.2 is officially outdated and unsupported!](https://www.ruby-lang.org/en/news/2018/06/20/support-of-ruby-2-2-has-ended/)
+- [Ruby 2.3 is officially outdated and unsupported!](https://www.ruby-lang.org/en/news/2019/03/31/support-of-ruby-2-3-has-ended/)
 
 :exclamation: Guard is currently accepting more maintainers. Please [read this](https://github.com/guard/guard/wiki/Maintainers) if you're interested in joining the team.
 
@@ -32,7 +36,7 @@ Before you file an issue, make sure you have read the _[known issues](#issues)_
 * File system changes handled by our awesome [Listen](https://github.com/guard/listen) gem.
 * Support for visual system notifications.
 * Huge eco-system with [more than 300](https://rubygems.org/search?query=guard-) Guard plugins.
-* Tested against the latest Ruby 2.3.x, 2.4.x, 2.5.x, 2.6.x, JRuby & Rubinius. See [`.travis-ci.yml`](https://github.com/guard/guard/blob/master/.travis.yml) for the exact versions.
+* Tested against the latest Ruby 2.4.x, 2.5.x, 2.6.x, JRuby & Rubinius. See [`.travis-ci.yml`](https://github.com/guard/guard/blob/master/.travis.yml) for the exact versions.
 
 ## Screencast