WTF-539 Add missing vendored dependencies back

vendor/github.com/gogo/protobuf/AUTHORS

@@ -0,0 +1,15 @@
+# This is the official list of GoGo authors for copyright purposes.
+# This file is distinct from the CONTRIBUTORS file, which
+# lists people.  For example, employees are listed in CONTRIBUTORS,
+# but not in AUTHORS, because the employer holds the copyright.
+
+# Names should be added to this file as one of
+#     Organization's name
+#     Individual's name <submission email address>
+#     Individual's name <submission email address> <email2> <emailN>
+
+# Please keep the list sorted.
+
+Sendgrid, Inc
+Vastech SA (PTY) LTD
+Walter Schulze <awalterschulze@gmail.com>