message: Update re2 to versioned release 2018-04-01
  This fix updates re2 to the most versioned release of 2018-04-01.
  (The version used in tf before was released last year and was not
  a versioned release.)
diff: @@ -155,12 +155,12 @@ def tf_workspace(path_prefix="", tf_repo_name=""):
  tf_http_archive(
  name = "com_googlesource_code_re2",
  urls = [
  - "https://mirror.bazel.build/github.com/google/re2/archive/26cd968b735e227361c9703683266f01e5df7857.tar.gz",
  - "https://github.com/google/re2/archive/26cd968b735e227361c9703683266f01e5df7857.tar.gz",
  + "https://mirror.bazel.build/github.com/google/re2/archive/2018-04-01.tar.gz",
  + "https://github.com/google/re2/archive/2018-04-01.tar.gz",
  ],
  - sha256 = "e57eeb837ac40b5be37b2c6197438766e73343ffb32368efea793dfd8b28653b",
  - strip_prefix = "re2-26cd968b735e227361c9703683266f01e5df7857",
  + sha256 = "2f945446b71336e7f5a2bcace1abcf0b23fbba368266c6a1be33de3de3b3c912",
  + strip_prefix = "re2-2018-04-01",
  )
  tf_http_archive(
  