Drop support for Ruby 2.3

It reached EOL on March 31, 2019, and no longer receives patches.

See https://www.ruby-lang.org/en/news/2019/03/31/support-of-ruby-2-3-has-ended/

.rubocop.yml

@@ -10,7 +10,7 @@ AllCops:
     - 'lib/yaml/*'
     - 'lib/**/*.sh'
   DisplayCopNames: true
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.4
 
 # Preferred codebase style ---------------------------------------------
 

.travis.yml

@@ -9,7 +9,6 @@ cache:
   bundler
 
 rvm:
-  - '2.3'
   - '2.4'
   - '2.5'
   - '2.6'

README.md

@@ -117,7 +117,7 @@ Man pages have been added. Checkout `man colorls`.
 
 [(Back to top)](#table-of-contents)
 
-1. Install Ruby (preferably, version > 2.2)
+1. Install Ruby (preferably, version > 2.3)
 2. Install the patched fonts of powerline nerd-font and/or font-awesome. Have a look at the [Nerd Font README](https://github.com/ryanoasis/nerd-fonts/blob/master/readme.md) for more installation instructions.
 
     *Note for `iTerm2` users - Please enable the Nerd Font at iTerm2 > Preferences > Profiles > Text > Non-ASCII font > Hack Regular Nerd Font Complete.*

colorls.gemspec

@@ -45,7 +45,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/athityakumar/colorls'
   spec.license       = 'MIT'
 
-  spec.required_ruby_version = '>= 2.3.0'
+  spec.required_ruby_version = '>= 2.4.0'
 
   spec.files         = `git ls-files -z`.split("\x0").reject do |f|
     f.match(%r{^(test|spec|features)/})