Revert jeet-reverse() to it's old version to fix #354

- Clean up old version a little
- Add extra tests to make sure context ratios output correct styles

If you experiment with [this Pen](http://codepen.io/clear-y/pen/xFkrv) you can see that commenting out the function breaks the layout.

As mentioned in #359 the tests fail seemingly due to number precision.

Is there any reason why this would cause issues with older/newer versions of Sass? Or was Hugo's optimisation just for our sake?