Bazel: Stop using native.git_repository

The native.git_repository method doesn't work in the latest version
of bazel, and causes the build to fail with:

  type 'struct' has no method git_repository()

Change-Id: Id6a57369b681c0afe811e9e3740b141fb7fb4653
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

tools/bazlets.bzl

@@ -1,10 +1,12 @@
+load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
+
 NAME = "com_googlesource_gerrit_bazlets"
 
 def load_bazlets(
         commit,
         local_path = None):
     if not local_path:
-        native.git_repository(
+        git_repository(
             name = NAME,
             remote = "https://gerrit.googlesource.com/bazlets",
             commit = commit,