Test against Ruby 2.2.6 on CircleCI

[koic]

Fixed following error when run bundle install.

bundle check --path=vendor/bundle || bundle install --path=vendor/bundle --jobs=4 --retry=3 
Bundler can't satisfy your Gemfile's dependencies.
Install missing gems with `bundle install`.
Fetching gem metadata from https://rubygems.org/........
Fetching version metadata from https://rubygems.org/..
Fetching dependency metadata from https://rubygems.org/.
Resolving dependencies...
ruby_dep-1.5.0 requires ruby version >= 2.2.5, which is incompatible with the
current version, ruby 2.2.4p230

cf. https://circleci.com/gh/ctran/annotate_models/257

[coveralls]

Coverage remained the same at 74.401% when pulling 2d8d63e on koic:ruby_2_2_6 into 1a64730 on ctran:develop.