Faster Circle CI build by caching Bundle install

[eijenson]

Issue

• close #580

Overview (Required)

• add steps of bundle save cache and bundle restore cache

Links

• Using cache build

[takahirom]

I have a question🙋 When changing Gemfile.lock Will ci work?

[eijenson]

Thanks for the question😀 When changed Gemfile.lock , Not use cache. result: https://circleci.com/gh/DroidKaigi/conference-app-2018/1205

I have a question. I want to delete 21fd478 and b15592e. Which way should I use Force Push or Revert?🤔

[takahirom]

You can use force push 👍 git reset --hard HEAD^ git push -f origin add_cache_for_bundle_install

[takahirom]

Thank you for faster build 👍