Drop support for Ruby 2.1

It reached EOL already and rubocop > 0.60 no longer supports it.

.travis.yml

@@ -5,7 +5,6 @@ cache:
   bundler
 
 rvm:
-  - '2.1'
   - '2.2'
   - '2.3'
   - '2.4'

colorls.gemspec

@@ -33,6 +33,8 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/athityakumar/colorls'
   spec.license       = 'MIT'
 
+  spec.required_ruby_version = '>= 2.2.0'
+
   spec.files         = `git ls-files -z`.split("\x0").reject do |f|
     f.match(%r{^(test|spec|features)/})
   end