Ruby 2.3.0 → 2.4.1 にした時に Rails が動かなくなった.

 $ rbenv install 2.4.1
ruby-build: definition not found: 2.4.1

The following versions contain `2.4.1' in the name:
  rbx-2.4.1

See all available versions with `rbenv install --list'.

If the version you need is missing, try upgrading ruby-build:

  cd /Users/_user_name_/.rbenv/plugins/ruby-build && git pull && cd -

 $ cd /Users/_user_name_/.rbenv/plugins/ruby-build && git pull && cd -

rubyをビルドするruby-buildのバージョンが最新版になってなかったみたい.

 $ bundle install
rbenv: bundle: command not found

The `bundle' command exists in these Ruby versions:
  2.3.0
  2.4.0

ふむふむ、2.4.1のbundlerが入っていないみたい!

 $ sudo gem install bundle

なるほど！Railsのinstallし直さないと行けない!!

 $ gem install rails
 $ rails s
=> Booting Puma
=> Rails 5.0.4 application starting in development on http://localhost:3000
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 3.9.1 (ruby 2.4.1-p111), codename: Private Caller
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://0.0.0.0:3000
Use Ctrl-C to stop

動いた！！