Remove sthulb/mine/multipart

This dep was meant as a stopgap for a patch in the stdlib of Go which I
wrote.

The patch was released in Go 1.7. We can now remove my fork and go back
to the stdlib

builtin/providers/template/datasource_cloudinit_config.go

@@ -6,13 +6,12 @@ import (
 	"encoding/base64"
 	"fmt"
 	"io"
+	"mime/multipart"
 	"net/textproto"
 	"strconv"
 
 	"github.com/hashicorp/terraform/helper/hashcode"
 	"github.com/hashicorp/terraform/helper/schema"
-
-	"github.com/sthulb/mime/multipart"
 )
 
 func dataSourceCloudinitConfig() *schema.Resource {