Use feature query for Firefox-specific <progress> styling

Created by: cvrebert

Given Firefox's relatively long-time support for feature queries, we should be able to replace https://github.com/twbs/bootstrap/blob/48e62fdf34c6f2a0a5c559e9b5e8318b7d86e654/scss/_progress.scss#L64 with a less hacky feature query, e.g. @supports (-moz-appearance:none) { ... } ala http://browserhacks.com/#hack-8e9b5504d9fda44ec75169381b3c3157