There's a reason and I should have added it to a comment. The `RUBY_ENGINE` constant is not set under MRI 1.8.7: it caused [this failure](https://travis-ci.org/mojombo/jekyll/jobs/14244547). An undefined constant does not return nil.

```
$ chruby ruby-1.8.7
$ irb
>> RUBY_ENGINE
NameError: uninitialized constant RUBY_ENGINE
        from (irb):1
        from :0
```