rails consoleで `require’: cannot load such file — readlineになる件

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CentOSにてreadline-develが入っていないと、rails consoleの実行で以下のようなエラーになります。

$ rails console
/usr/local/rbenv/versions/2.1.2/lib/ruby/2.1.0/irb/completion.rb:9:in `require': cannot load such file -- readline (LoadError)
from /usr/local/rbenv/versions/2.1.2/lib/ruby/2.1.0/irb/completion.rb:9:in `'
from /usr/local/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/railties-4.1.2/lib/rails/commands/console.rb:3:in `require'
from /usr/local/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/railties-4.1.2/lib/rails/commands/console.rb:3:in `'
from /usr/local/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/railties-4.1.2/lib/rails/commands/commands_tasks.rb:128:in `require'
from /usr/local/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/railties-4.1.2/lib/rails/commands/commands_tasks.rb:128:in `require_command!'
from /usr/local/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/railties-4.1.2/lib/rails/commands/commands_tasks.rb:59:in `console'
from /usr/local/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/railties-4.1.2/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
from /usr/local/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/railties-4.1.2/lib/rails/commands.rb:17:in `'
from bin/rails:4:in `require'
from bin/rails:4:in `'

yumでインストールしてもよいですが、ここではrb-readlineのGemにて解決しましょう。
まず、Gemfileに以下を記述します。

gem 'rb-readline'

インストールを実行して、

$ bundle install

再度、rails consoleを実行します。

$ rails console
Loading development environment (Rails 4.1.2)
irb(main):001:0>

これでOKです。

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