test: add minitest to dev dependencies

This allows us to be independent from bundled versions and to use the
new features that minitest has.

Author: rmehner

transloadit.gemspec

@@ -27,11 +27,13 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'mime-types'
 
   gem.add_development_dependency 'rake'
+  gem.add_development_dependency 'minitest'
   gem.add_development_dependency 'simplecov'
 
   gem.add_development_dependency 'vcr'
   gem.add_development_dependency 'webmock'
 
   gem.add_development_dependency 'yard'
   gem.add_development_dependency 'kramdown' # for YARD rdoc formatting
+
 end