Update rubocop requirement from ~> 0.40.0 to ~> 0.50.0

Updates the requirements on [rubocop](https://github.com/rubocop-hq/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop-hq/rubocop/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop-hq/rubocop/commits/v0.50.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

sensu-plugins-mysql.gemspec

@@ -44,6 +44,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake',                      '~> 10.0'
   s.add_development_dependency 'redcarpet',                 '~> 3.2'
   s.add_development_dependency 'rspec',                     '~> 3.1'
-  s.add_development_dependency 'rubocop',                   '~> 0.40.0'
+  s.add_development_dependency 'rubocop',                   '~> 0.50.0'
   s.add_development_dependency 'yard',                      '~> 0.8'
 end