fix #147 - plugin not loading all chapters

Why it's using the **if (this.spinePos < this.spine.length - 1)**, if I'm using **array length** and the operator **<**, do I need to the **- 1** ? 
This change fix the issue #147
Whoops, this still needs to be -1, since 1 is added  on line 3084 - updated in src/epub.js on master