`Sundown` commit fix up in order to resolve `inline` linking problem in MSVC

Defining inline as empty in [`buffer.h`](https://github.com/apiaryio/sundown/blob/zdne/listitem_maps/src/buffer.h#L31) was causing redefinition linking problem in `snowcrash` after the implementation of refactor integration. this PR change the head of sundown to correct the head.
cc. @zdne