gems: include rubocop-rake and fix linting

.rubocop.yml

@@ -1,3 +1,6 @@
+require:
+- rubocop-rake
+
 AllCops:
   TargetRubyVersion: '2.5'
   NewCops: enable

Rakefile

@@ -23,4 +23,5 @@ end
 
 task default: [:rubocop, :spec, 'bundle:audit', :solargraph]
 
+desc 'Run all tasks desired for CI'
 task ci: ['solargraph:init', :default]

openvpn-status-web.gemspec

@@ -38,5 +38,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake'
   s.add_development_dependency 'rspec'
   s.add_development_dependency 'rubocop', '~> 1.6.1'
+  s.add_development_dependency 'rubocop-rake', '~> 0.5.1'
   s.add_development_dependency 'solargraph', ' ~> 0.40.0'
 end