pry起動時にエラーが出た場合の対処法

pryを起動しようとしたところ、下記のエラーが出力された。
環境：Ubuntu

```
$ pry
Sorry, you can't use Pry without Readline or a compatible library.
Possible solutions:
 * Rebuild Ruby with Readline support using `--with-readline`
 * Use the rb-readline gem, which is a pure-Ruby port of Readline
 * Use the pry-coolline gem, a pure-ruby alternative to Readline
/home/vagrant/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- readline (LoadError)
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/config/default.rb:151:in `lazy_readline'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/config/default.rb:6:in `block in <class:Default>'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/config/default.rb:125:in `instance_eval'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/config/default.rb:125:in `block (2 levels) in <class:Default>'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/config/behavior.rb:54:in `public_send'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/config/behavior.rb:54:in `method_missing'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/history.rb:19:in `restore_default_behavior'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/history.rb:14:in `initialize'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/pry_class.rb:33:in `new'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/pry_class.rb:33:in `history'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/pry_class.rb:222:in `load_history'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/pry_class.rb:129:in `initial_session_setup'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/cli.rb:206:in `block in <top (required)>'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/cli.rb:83:in `call'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/cli.rb:83:in `block in parse_options'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/cli.rb:83:in `each'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/lib/pry/cli.rb:83:in `parse_options'
	from /home/vagrant/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/pry-0.10.1/bin/pry:16:in `<top (required)>'
	from /home/vagrant/.rbenv/versions/2.1.1/bin/pry:23:in `load'
	from /home/vagrant/.rbenv/versions/2.1.1/bin/pry:23:in `<main>'  
```

どうやら、Readlineというのがないみたい。
なので、インストール。

```
$ gem install rb-readline
Fetching: rb-readline-0.5.2.gem (100%)
Successfully installed rb-readline-0.5.2
Parsing documentation for rb-readline-0.5.2
Installing ri documentation for rb-readline-0.5.2
Done installing documentation for rb-readline after 1 seconds
1 gem installed
```

起動するでしょうか。。。

```
$ pry
[1] pry(main)>
```

やった :satisfied: