Changed default tags to <mark> (from <strong>) in the highlight_phrase() method in text helper

I've changed the default tag for the `highlight_phrase()` method in the text helper from `<strong>` to `<mark>`.

The `<mark>` tag semantically represents highlighted text in HTML5. The spec details can be found here http://www.w3.org/TR/html5/the-mark-element.html#the-mark-element