Upgrade Go Bazel rules to the latest version

bazel 1.7 has a breaking change that requires an update of the go rules
(bazelbuild/bazel#7793).

Change-Id: I43c375315cda7112df43f93b0c274aecdf7171ab

WORKSPACE

@@ -83,8 +83,11 @@ closure_repositories(
 # Golang support for PolyGerrit local dev server.
 http_archive(
     name = "io_bazel_rules_go",
-    sha256 = "6776d68ebb897625dead17ae510eac3d5f6342367327875210df44dbe2aeeb19",
-    url = "https://github.com/bazelbuild/rules_go/releases/download/0.17.1/rules_go-0.17.1.tar.gz",
+    sha256 = "f04d2373bcaf8aa09bccb08a98a57e721306c8f6043a2a0ee610fd6853dcde3d",
+    urls = [
+        "https://storage.googleapis.com/bazel-mirror/github.com/bazelbuild/rules_go/releases/download/0.18.6/rules_go-0.18.6.tar.gz",
+        "https://github.com/bazelbuild/rules_go/releases/download/0.18.6/rules_go-0.18.6.tar.gz",
+    ],
 )
 
 load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")