Make ratings stars 20px to match dashicons native size so they're crisp on all screens. See #26170, props melchoyce.
Built from https://develop.svn.wordpress.org/trunk@26393


git-svn-id: http://core.svn.wordpress.org/trunk@26293 1a063a9b-81f0-0310-95a4-ce76da25c4cd