Remove incorrect ARIA roles

[kerhong]

This library currently uses invalid ARIA roles, instead use class names to select items.

List of valid roles: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques

Alternative: find some more appropriate roles for these elements.

This could be a breaking change if users are accessing or styling elements using these incorrect roles.

Fixes #180

[nmackey]

For anyone that finds this in the future I have forked this repo and created accessible-nprogress that adds accessibility support for this library. - https://github.com/nmackey/accessible-nprogress