No -webkit and -moz vendor prefixes for @viewport

Created by: Ryuno-Ki

The documentation at https://github.com/twbs/bootstrap/blob/10606a73bcbfc6c723d60e2a1069c921afff0d2a/docs/_includes/getting-started/browser-device-support.html#L154 is wrong.

According to Can I Use? and WebPlatform.org the only supported vendor prefixes are

@-ms-viewport       { width: device-width; }
@-o-viewport        { width: device-width; }
@viewport           { width: device-width; }

gulp-csslint threw a parser error in my project.