Migrate to supported version of http_archive

Fixes #4727.

WORKSPACE

@@ -1,4 +1,5 @@
 workspace(name = "com_google_protobuf")
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 new_local_repository(
     name = "submodule_gmock",
@@ -6,7 +7,7 @@ new_local_repository(
     build_file = "third_party/googletest/BUILD.bazel"
 )
 
-new_http_archive(
+http_archive(
     name = "six_archive",
     build_file = "six.BUILD",
     sha256 = "105f8d68616f8248e24bf0e9372ef04d3cc10104f1980f54d57b2ce73a5ad56a",