IToastPreset: add missing toastClass() method

Add toastClass() method to IToastPreset according to the docs: https://material.angularjs.org/latest/api/service/$mdToast

angular-material/angular-material.d.ts

@@ -160,6 +160,7 @@ declare namespace angular.material {
         hideDelay(delay: number): T;
         position(position: string): T;
         parent(parent?: string|Element|JQuery): T; // default: root node
+        toastClass(toastClass: string): T;
     }
 
     interface ISimpleToastPreset extends IToastPreset<ISimpleToastPreset> {