it seems letsencrypt is not installable on an up-to-date Jessie system:
The "letsencrypt" package is replaced by "certbot" (the upstream project was renamed); it looks like "certbot" does have Provides: letsencrypt for compatibility, but I think the old binaries from the letsencrypt package need to be removed before this will work.
That said, just installing "certbot" directly instead of "letsencrypt" should resolve your issue.