Update build file locations.

Fixes https://github.com/bazelbuild/bazel/issues/5383.

WORKSPACE

@@ -4,12 +4,12 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 new_local_repository(
     name = "submodule_gmock",
     path = "third_party/googletest",
-    build_file = "third_party/googletest/BUILD.bazel"
+    build_file = "@//:third_party/googletest/BUILD.bazel"
 )
 
 http_archive(
     name = "six_archive",
-    build_file = "six.BUILD",
+    build_file = "@//:six.BUILD",
     sha256 = "105f8d68616f8248e24bf0e9372ef04d3cc10104f1980f54d57b2ce73a5ad56a",
     urls = ["https://pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz#md5=34eed507548117b2ab523ab14b2f8b55"],
 )