Log Message:
Updated p5-YAML to 1.21.

1.21 Fri Dec 23 21:19:15 CET 2016
 - Apply PR/171 (fixes issue/109) @perlpunk++
 - No more "used only once" warnings for $YAML::Indent etc.
 - Apply PR/170 (fixes issue/131) hiratara@cpan.org++
 - Empty mapping value at the end resolves to null (was becoming empty
   string)
 - Apply PR/169 (PR/119) patrick.allen.higgins@gmail.com++
 - Output key in warning when duplicate key was found
 - Apply PR/157 and PR/168 (@lameventanas++ @perlpunk++)
 - Allow reading and writing to IO::Handle