message-signature has been renamed to mu4e-compose-signature 

@see https://github.com/djcb/mu/commit/9c8101d949d66401415f2d22cbb7e07d743f45a5

BTY, thank you for your config,I've learnt and stolen a lot.:-)
Merge pull request #2 from mikulely/patch-1

message-signature has been renamed to mu4e-compose-signature