Upgrade guava to 21.0

Change-Id: I159c6d3a42e5166f93c6eb410018ce3297876e50

lib/guava.bzl

diff --git a/lib/guava.bzl b/lib/guava.bzl
index 7ba9079f..c71379e 100644
--- a/lib/guava.bzl
+++ b/lib/guava.bzl
@@ -1,5 +1,5 @@
-GUAVA_VERSION = "20.0"
+GUAVA_VERSION = "21.0"
 
-GUAVA_BIN_SHA1 = "89507701249388e1ed5ddcf8c41f4ce1be7831ef"
+GUAVA_BIN_SHA1 = "3a3d111be1be1b745edfa7d91678a12d7ed38709"
 
 GUAVA_DOC_URL = "https://google.github.io/guava/releases/" + GUAVA_VERSION + "/api/docs/"