Migrate codecov uploader

- [codecov-ruby](https://github.com/codecov/codecov-ruby) is deprecated.
- Followed the [migration guide](https://docs.codecov.com/docs/deprecated-uploader-migration-guide#ruby-uploader) and migrated to the new uploader.

aasm.gemspec

@@ -23,8 +23,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec', ">= 3"
   s.add_development_dependency 'generator_spec'
   s.add_development_dependency 'appraisal'
+  s.add_development_dependency "simplecov"
   s.add_development_dependency "simplecov-cobertura"
-  s.add_development_dependency "codecov", ">= 0.1.21"
 
   # debugging
   # s.add_development_dependency 'debugger'