fix(radioGroup): fix render call happing before radioGroup init

closes #1141

Author: rschmukler

src/components/radioButton/radioButton.js

@@ -56,7 +56,7 @@ function mdRadioGroupDirective($mdUtil, $mdConstant, $mdTheming) {
     restrict: 'E',
     controller: ['$element', RadioGroupController],
     require: ['mdRadioGroup', '?ngModel'],
-    link: linkRadioGroup
+    link: { pre: linkRadioGroup }
   };
 
   function linkRadioGroup(scope, element, attr, ctrls) {