.hint class isn't required. Can use [data-hint]

The `.hint` class really isn't needed, as `[data-hint]` is a valid selector, making `.hint` redundant. However, I've kept `.hint` in for backwards compatibility, and for browsers that may not support attribute selectors. *(are there any?)*

I also fixed some issues that were preventing me from deploying. These are:

 - The use of old-style mixin includes
 - The use of a version of `grunt-sass` that couldn't be found.

Now, to create a hint, you just need this.

```html
<div class="hint--top" data-hint="I'm a hint!">Lorem sit amet...</div>
```