Add rake task for style and tests

Instead of remembering all the commands it would be nice to have rake task to run tests and style checkers
closes #164

.ruby-version

@@ -0,0 +1 @@
+2.4.0

Rakefile

@@ -10,3 +10,20 @@ namespace :gem do
     system 'gem build trollolo.gemspec'
   end
 end
+
+require 'rubocop/rake_task'
+require 'rspec/core/rake_task'
+
+task default: %i[rubocop spec]
+
+desc 'Run tests'
+task :spec do
+  RSpec::Core::RakeTask.new
+end
+
+desc 'Run rubocop'
+task :rubocop do
+  RuboCop::RakeTask.new do |t|
+    t.options = ['--display-cop-names']
+  end
+end