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/golang.org/x/net/context/context.go

@@ -34,7 +34,7 @@
 //
 // See http://blog.golang.org/context for example code for a server that uses
 // Contexts.
-package context
+package context // import "golang.org/x/net/context"
 
 import (
 	"errors"

vendor/golang.org/x/net/context/ctxhttp/ctxhttp.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Package ctxhttp provides helper functions for performing context-aware HTTP requests.
-package ctxhttp
+package ctxhttp // import "golang.org/x/net/context/ctxhttp"
 
 import (
 	"io"