Reference rules_go using reliable http_archive() (#18)

Fetching via git is slow, buggy, and wastes disk space.

CONTRIBUTORS

diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 723b5a0..f21890f 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -11,3 +11,4 @@
 Paul Bethe <pbethe@google.com>
 Laurent Le Brun <laurentlb@google.com>
 Nilton Volpato <nilton@google.com>
+Justine Alexandra Roberts Tunney <jart@google.com>

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 6357a75..722ab4d 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,10 +1,12 @@
 workspace(name = "com_github_bazelbuild_buildifier")
 
-git_repository(
+http_archive(
     name = "io_bazel_rules_go",
-    remote = "https://github.com/bazelbuild/rules_go.git",
-    tag = "0.2.0",
+    sha256 = "0c0ec7b9c7935883cbfb2df48fbf524e857859a5c05ae1b24d5442956e6bb5e8",
+    strip_prefix = "rules_go-0.2.0",
+    url = "http://bazel-mirror.storage.googleapis.com/github.com/bazelbuild/rules_go/archive/0.2.0.tar.gz",
 )
+
 load("@io_bazel_rules_go//go:def.bzl", "go_repositories")
 
 go_repositories()