fix(alert): adjust check for close attribute

- Adjust check due to change in Angular 1.4 where the presence of attribute gives key value of `undefined`, causing variable to contain incorrect truthiness

src/alert/alert.js

@@ -1,7 +1,7 @@
 angular.module('ui.bootstrap.alert', [])
 
 .controller('AlertController', ['$scope', '$attrs', function ($scope, $attrs) {
-  $scope.closeable = 'close' in $attrs;
+  $scope.closeable = $attrs.close;
   this.close = $scope.close;
 }])