Use Bazel mirror to download externals

[jart]

I've mirrored your files for guaranteed reliability, courtesy of Google Cloud Storage.

In the future, you can mirror these files too with the following command. Although you need to ask @damienmg for write access to the bucket. Or just email me the URL you want mirrored and I'll happily do it.

bzmirror() {
  local url="${1:?url}"
  if [[ "${url}" =~ ^([^:]+):([^:]+):([^:]+)$ ]]; then
    url="http://repo1.maven.org/maven2/${BASH_REMATCH[1]//.//}/${BASH_REMATCH[2]}/${BASH_REMATCH[3]}/${BASH_REMATCH[2]}-${BASH_REMATCH[3]}.jar"
  fi
  local dest="gs://bazel-mirror/${url#http*//}"
  local desturl="http://bazel-mirror.storage.googleapis.com/${url#http*//}"
  local name="$(basename "${dest}")"
  wget -O "/tmp/${name}" "${url}" || return 1
  gsutil cp -a public-read "/tmp/${name}" "${dest}" || return 1
  gsutil setmeta -h 'Cache-Control:public, max-age=31536000' "${dest}" || return 1
  curl -I "${desturl}"
  echo
  sha256sum "/tmp/${name}"
  echo "${desturl}"
  rm "/tmp/${name}" || return 1
}

[davidzchen]

LGTM. Thanks! Can you open a separate PR for updating the README to use http_archive?