[WIP] Enable to toggle whether to install `Ruby` or not.

Introduce an attribute to determine whether or not to install `Ruby`.
Introduce an attribute to determine whether to install `Ruby` or not.

Introduce the following attribute:

```
default['ruby']['install'] = true
```
Follow the `foodcritic`'s advice and access node attributes in a consistent manner.

In order to follow the advice below, try some modification:

```
bundle exec foodcritic --epic-fail "~FC048" ./tmp/foodcritic/site-cookbooks
FC019: Access node attributes in a consistent manner: tmp/foodcritic/site-cookbooks/ruby/providers/setup.rb:76
FC048: Prefer Mixlib::ShellOut: ./tmp/foodcritic/site-cookbooks/jenkins/recipes/env.rb:19
FC048: Prefer Mixlib::ShellOut: ./tmp/foodcritic/site-cookbooks/ruby/providers/setup.rb:112
FC048: Prefer Mixlib::ShellOut: ./tmp/foodcritic/site-cookbooks/ruby/providers/setup.rb:122
FC048: Prefer Mixlib::ShellOut: ./tmp/foodcritic/site-cookbooks/ruby/providers/setup.rb:136
FC048: Prefer Mixlib::ShellOut: ./tmp/foodcritic/site-cookbooks/sensu-custom/recipes/client_settings.rb:10
rake aborted!
```
Merge pull request #123 from kazu634/ruby-toggle-installation

Enable to toggle whether to install `Ruby` or not.