Update minitest requirement from ~> 5.4.0 to ~> 5.11.3

Updates the requirements on [minitest](https://github.com/seattlerb/minitest) to permit the latest version.
- [Release notes](https://github.com/seattlerb/minitest/releases)
- [Changelog](https://github.com/seattlerb/minitest/blob/master/History.rdoc)
- [Commits](https://github.com/seattlerb/minitest/commits)

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

alchemy-api-rb.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency             'faraday',             '~> 0.9.0'
   gem.add_dependency             'excon',               '~> 0.28'
 
-  gem.add_development_dependency 'minitest',            '~> 5.4.0'
+  gem.add_development_dependency 'minitest',            '~> 5.11.3'
   gem.add_development_dependency 'pry',                 '~> 0.10.0'
   gem.add_development_dependency 'rake',                '~> 10.3.0'
   gem.add_development_dependency 'minitest-reporters',  '~> 1.0.0'