Fixed mangahere issue with out of order next chapter

Fixed mangahere chapter listing by ensuring sorting of chapters. The name returned as the next chapter could be a previously uploaded one. i.e. 1-400 with next chapter being 240. This happened with Hajime no Ippo for example as the next chapter was pointing to 1000.5 instead of 1053.

Not sure if this is the best solution so feel free to pitch in if necessary