vim/syntastic: show "Documentation warning" as a style warning

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
vim/syntastic: show errors on EOF on the last line

Requires to update syntastic, uses the `guards` function introduced in
september 2014.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>