Upgrade guava to 23.6-jre

Change-Id: I276ff98b51e7af101ad2555921197dca34dc0bf9

lib/guava.bzl

diff --git a/lib/guava.bzl b/lib/guava.bzl
index 551e2c5..0aa3cf7a 100644
--- a/lib/guava.bzl
+++ b/lib/guava.bzl
@@ -1,5 +1,5 @@
-GUAVA_VERSION = "23.5-jre"
+GUAVA_VERSION = "23.6-jre"
 
-GUAVA_BIN_SHA1 = "e9ce4989adf6092a3dab6152860e93d989e8cf88"
+GUAVA_BIN_SHA1 = "c0b638df79e7b2e1ed98f8d68ac62538a715ab1d"
 
 GUAVA_DOC_URL = "https://google.github.io/guava/releases/" + GUAVA_VERSION + "/api/docs/"