Switch to using the dep tool and update all the dependencies

vendor/google.golang.org/appengine/internal/mail/mail_service.proto

@@ -0,0 +1,45 @@
+syntax = "proto2";
+option go_package = "mail";
+
+package appengine;
+
+message MailServiceError {
+  enum ErrorCode {
+    OK = 0;
+    INTERNAL_ERROR = 1;
+    BAD_REQUEST = 2;
+    UNAUTHORIZED_SENDER = 3;
+    INVALID_ATTACHMENT_TYPE = 4;
+    INVALID_HEADER_NAME = 5;
+    INVALID_CONTENT_ID = 6;
+  }
+}
+
+message MailAttachment {
+  required string FileName = 1;
+  required bytes Data = 2;
+  optional string ContentID = 3;
+}
+
+message MailHeader {
+  required string name = 1;
+  required string value = 2;
+}
+
+message MailMessage {
+  required string Sender = 1;
+  optional string ReplyTo = 2;
+
+  repeated string To = 3;
+  repeated string Cc = 4;
+  repeated string Bcc = 5;
+
+  required string Subject = 6;
+
+  optional string TextBody = 7;
+  optional string HtmlBody = 8;
+
+  repeated MailAttachment Attachment = 9;
+
+  repeated MailHeader Header = 10;
+}