Upgrade to a new Bazel version

This one has better performance and various new features. It also
prevents us from falling too far behind upstream.

Change-Id: I3b8e1c5bda886ea2e990523af9e6de2434c784be
diff --git a/aos/downloader/downloader.bzl b/aos/downloader/downloader.bzl
index e5179c9..0219cf7 100644
--- a/aos/downloader/downloader.bzl
+++ b/aos/downloader/downloader.bzl
@@ -61,7 +61,7 @@
   attrs = {
     '_downloader': attr.label(
       executable = True,
-      cfg = HOST_CFG,
+      cfg = 'host',
       default = Label('//aos/downloader'),
     ),
     'start_srcs': attr.label_list(