Fixed MangaRock.js userscript

[c0d3rman]

Description

Fixed selectors to work with the new MangaRock redesign. Fixes issue #417

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] My changes generate no new warnings

[DakuTree]

Looks good, merging.