chore(build): use third_party.go

use the third_party.go project to replace our update script. This
requires moving a few things around and gets rid of a few annoying bugs:

- You can now bump individual packages
- A new src directory isn't created on build
- Less shell scripting!
- Things get built into ./bin/

third_party/github.com/gorilla/mux/README.md

@@ -1,7 +0,0 @@
-mux
-===
-[![Build Status](https://travis-ci.org/gorilla/mux.png?branch=master)](https://travis-ci.org/gorilla/mux)
-
-gorilla/mux is a powerful URL router and dispatcher.
-
-Read the full documentation here: http://www.gorillatoolkit.org/pkg/mux