NoMethodError: undefined method `empty?` on Fixnum

Looks like we aren't so good about checking types...

```
             Fatal: NoMethodError
                    undefined method `empty?' for 428626:Fixnum
+ .bundle/ruby/2.3.0/gems/jekyll-gist-1.4.0/lib/jekyll-gist/gist_tag.rb:50:in `gist_script_tag'
```