homebrew-cask: update according to new sharding

Update to correctly publish casks to homebrew/homebrew-cask
based on sharding [1].

[1]: Homebrew/homebrew-cask#152603

src/homebrew.ts

@@ -103,7 +103,8 @@ export class Tap {
   }
 
   async getCaskAsync(name: string): Promise<Package> {
-    const filePath = `Casks/${name}.rb`;
+    const prefix = this.repo.name === 'homebrew-cask' ? 'Casks/g' : 'Casks';
+    const filePath = `${prefix}/${name}.rb`;
     return this.getPackageAsync(filePath);
   }