Add check to print Ruby version

Display the version of Ruby that's running the app. Useful when upgrading the version of Ruby running on a server, to verify that you're up and running with the newly installed version.

```ruby
# config/initializers/okcomputer.rb
OKComputer::Registry.register "ruby_version", OKComputer::RubyVersionCheck.new
```

```
✓ curl http://example.com/okcomputer/all.txt | grep 'ruby_version'
ruby_version: Ruby 2.0.0-p353
```