Bump version to 1.6.0

Changes from 1.5.0:
  * Added STDIN support to maktaba#syscall.
  * Fix ftplugin files not being entered in some buffers.
  * Fix maktaba#path#Split to respect slashes on Windows.
  * Omit trailing slashes from rtp entries in maktaba#rtp#Join().
  * Fix maktaba#autocmd#ClearGroup() to not end current augroup.