fix(tabs): change missed md-active to md-tab-active (#1044)

Author: tlein

src/components/tabs/tab-group.scss

@@ -60,7 +60,7 @@ $md-tab-bar-height: 48px !default;
   box-sizing: border-box;
   flex-grow: 1;
   flex-shrink: 1;
-  &.md-active {
+  &.md-tab-active {
     display: block;
   }
 }