Test on Rubinius 1.9 mode

The specs pass on Rubinius in 1.9 mode except one:

<pre>
expected "Liquid error: method 'resize': given 1, expected 2" to include "Liquid error: wrong number of arguments"
</pre>

I would fix this if I knew a good way to do so. Basically, it's an implementation-specific error message. I think you may agree that Rubinius has a better message. :)

Anyway, would be awesome to have this rad project testing on Rubinius as well!