use vertical-align: middle; in radio group also

As in [button
group](https://github.com/iview/iview/blob/v2.0.0/src/styles/mixins/butt
on.less#L56) this makes grouped radio buttons to have same vertical
align.

src/styles/components/radio.less

@@ -6,6 +6,7 @@
 .@{radio-group-prefix-cls} {
     display: inline-block;
     font-size: @font-size-small;
+    vertical-align: middle;
     &-vertical{
         .@{radio-prefix-cls}-wrapper {
             display: block;