Update CircleCI build to use Mage

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Also, since this is includes updating a docker image might as well update Goreleaser + Go (1.9.1).

[bep]

This still doesn't work:

Error: mage.go:5:9: could not import fmt (cannot find package "fmt" in any of:
	/Users/finchnat/src/github.com/golang/go/src/fmt (from $GOROOT)
	/go/src/fmt (from $GOPATH))
root@a7699ceb0bec:/go# mage -version
Mage Build Tool 1.0.1
Build Date: 2017-10-16T03:55:08Z
Commit: e66e3df1f2a53233ad48e5e874a0e532c65592dd
root@a7699ceb0bec:/go#

mage.go is a very simple build file and it, obviously, also fails for more complex scenarios:

// +build mage

package main

import ("fmt")

func Build() {
    fmt.Println("hi")
}

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