Move to go modules. (#704)

May be a bit of a work in progress before you accept and I'm sure
I've missed multiple things, but wanted to kick things off with the
migration to go modules.

Everything compiles and works afaik, but obviously requires go 1.11.

CONTRIBUTING.md

@@ -2,14 +2,12 @@
 
 Before contributing to Up you'll need a few things:
 
-- Install [Golang 1.9](https://golang.org/dl/) for that Go thing if you don't have it
-- Install [golang/dep](https://github.com/golang/dep) for dependency management
+- Install [Golang 1.11](https://golang.org/dl/) for that Go thing if you don't have it
 
 The following are optional:
 
 - Install [pointlander/peg](https://github.com/pointlander/peg) if you're working on the log grammar
 - Install [jteeuwen/go-bindata](https://github.com/jteeuwen/go-bindata) if you need to bake `up-proxy` into `up`
-- Or run `make install.deps` to grab these tools
 
 ## Setup
 
@@ -25,18 +23,6 @@ Change into the project:
 $ cd $GOPATH/src/github.com/apex/up
 ```
 
-Grab the dependencies:
-
-```
-$ dep ensure
-```
-
-Grab the dev dependencies:
-
-```
-make install.deps
-```
-
 ## Testing
 
 ```