gem: bump minimum required Ruby version to Ruby 2.5 (#14)

.rubocop.yml

@@ -1,5 +1,6 @@
 AllCops:
-  TargetRubyVersion: '2.3'
+  TargetRubyVersion: '2.5'
+  NewCops: enable
 
 Layout/EmptyLineAfterGuardClause:
   Enabled: false

.travis.yml

@@ -5,8 +5,6 @@ rvm:
 - 2.7
 - 2.6
 - 2.5
-- 2.4
-- 2.3
 
 script:
 - bundle exec rake travis

openvpn-status-web.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |s|
   s.executables = ['openvpn-status-web']
   s.extra_rdoc_files = Dir['README.md', 'LICENSE']
 
-  s.required_ruby_version = '>= 2.3'
+  s.required_ruby_version = '>= 2.5'
 
   s.add_runtime_dependency 'metriks'
   s.add_runtime_dependency 'rack', '~> 2.0'
@@ -36,6 +36,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rack-test'
   s.add_development_dependency 'rake'
   s.add_development_dependency 'rspec'
-  s.add_development_dependency 'rubocop', '~> 0.81.0'
+  s.add_development_dependency 'rubocop', '~> 0.82.0'
   s.add_development_dependency 'solargraph'
 end