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

sshd/server.go

@@ -2,10 +2,11 @@ package sshd
 
 import (
 	"fmt"
+	"net"
+
 	"github.com/armon/go-radix"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/crypto/ssh"
-	"net"
 )
 
 type SSHServer struct {