fix #1319 -- rebuild posts when metadata changes

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
use append in both cases

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Merge pull request #1382 from getnikola/depend-on-source-files

fix #1319 -- rebuild posts when metadata changes