Replace godep with vndr

Vndr has a simpler configuration and allows pointing to forked
packages. Additionally other docker projects are now using
vndr making vendoring in distribution more consistent.

Updates letsencrypt to use fork.
No longer uses sub-vendored packages.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

vendor/github.com/docker/libtrust/CONTRIBUTING.md

@@ -1,13 +0,0 @@
-# Contributing to libtrust
-
-Want to hack on libtrust? Awesome! Here are instructions to get you
-started.
-
-libtrust is a part of the [Docker](https://www.docker.com) project, and follows
-the same rules and principles. If you're already familiar with the way
-Docker does things, you'll feel right at home.
-
-Otherwise, go read
-[Docker's contributions guidelines](https://github.com/docker/docker/blob/master/CONTRIBUTING.md).
-
-Happy hacking!

vendor/github.com/docker/libtrust/MAINTAINERS

@@ -1,3 +0,0 @@
-Solomon Hykes <solomon@docker.com>
-Josh Hawn <josh@docker.com> (github: jlhawn)
-Derek McGowan <derek@docker.com> (github: dmcgowan)

vendor/github.com/docker/libtrust/README.md

@@ -1,18 +0,0 @@
-# libtrust
-
-Libtrust is library for managing authentication and authorization using public key cryptography.
-
-Authentication is handled using the identity attached to the public key.
-Libtrust provides multiple methods to prove possession of the private key associated with an identity.
- - TLS x509 certificates
- - Signature verification
- - Key Challenge
-
-Authorization and access control is managed through a distributed trust graph.
-Trust servers are used as the authorities of the trust graph and allow caching portions of the graph for faster access.
-
-## Copyright and license
-
-Code and documentation copyright 2014 Docker, inc. Code released under the Apache 2.0 license.
-Docs released under Creative commons.
-