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-service/main.go

@@ -3,9 +3,10 @@ package main
 import (
 	"flag"
 	"fmt"
+	"os"
+
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula"
-	"os"
 )
 
 // A version string that can be set with

cmd/nebula-service/service.go

@@ -2,12 +2,12 @@ package main
 
 import (
 	"fmt"
-	"github.com/sirupsen/logrus"
 	"log"
 	"os"
 	"path/filepath"
 
 	"github.com/kardianos/service"
+	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula"
 )