chore(typings): revert to 6c32d10

PowerPartial support by TypeScript v2.8.0+

Author: waitingsong

index.d.ts

@@ -420,9 +420,9 @@ declare module 'egg' {
     } & PlainObject;
 
     view: {
-      root?: string;
-      cache?: boolean;  // default true
-      defaultExtension?: string;
+      root: string;
+      cache: boolean;
+      defaultExtension: string;
       defaultViewEngine: string;
       mapping: PlainObject<string>;
     };