[WIP] Make Symbols Comparable

I recently noticed that, in Rubinius, `Symbol` is not [`Comparable`](http://ruby-doc.org/core-2.1.0/Comparable.html). This feature has been in MRI since version 1.9.

###### MRI 1.9 and greater
```ruby
:baz.is_a?(:Comparable) #=> true
:baz.between?(:bar, :foo) #=> true
```