Post sort fix

[KernelPanicAUS]

I've noticed that the 'next' and 'previous' page attributes were back to front.
This was due to how the posts were sorted in the post generator.
Tests have been updated accordingly.

[coveralls]

Coverage remained the same at 97.132% when pulling a2a8083 on KernelPanicAUS:post_sort_fix into dcd238f on hexojs:master.

[NoahDragon]

Could you please remove yarn.lock or add it to the gitignore?

[KernelPanicAUS]

Done :)

[coveralls]

Coverage remained the same at 97.132% when pulling fcf38b0 on KernelPanicAUS:post_sort_fix into dcd238f on hexojs:master.

[coveralls]

Coverage remained the same at 97.132% when pulling fcf38b0 on KernelPanicAUS:post_sort_fix into dcd238f on hexojs:master.

[NoahDragon]

After rethinking about this issue, I think this may be varied based on the perspective of view on previous and next.

Let's say we have several posts from newest to oldest:

New<--------------------------------->Old
                     Next<--|-->Prev                         `posts.sort('date')`
                     Prev<--|-->Next                         `posts.sort('-date')`

The Prev and Next do not have any time directions, either way is okay.

However, posts.sort('-date') has used since Hexo 2.0. I'm going to revert this change as it is starting to break existing themes.