enforce the use of goimports (#248)

* enforce the use of goimports

Instead of enforcing `gofmt`, enforce `goimports`, which also asserts
a separate section for non-builtin packages.

* run `goimports` everywhere

* exclude generated .pb.go files

cmd/nebula-cert/print_test.go

@@ -2,12 +2,13 @@ package main
 
 import (
 	"bytes"
-	"github.com/slackhq/nebula/cert"
-	"github.com/stretchr/testify/assert"
 	"io/ioutil"
 	"os"
 	"testing"
 	"time"
+
+	"github.com/slackhq/nebula/cert"
+	"github.com/stretchr/testify/assert"
 )
 
 func Test_printSummary(t *testing.T) {