Upgrade Truth to 0.31

Change-Id: I94c9d31c86b9d9196a74baa35614b319f307b429

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index fb85d72..385920e 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -93,8 +93,8 @@
 
 maven_jar(
     name = "truth",
-    artifact = "com.google.truth:truth:0.28",
-    sha1 = "0a388c7877c845ff4b8e19689dda5ac9d34622c4",
+    artifact = "com.google.truth:truth:0.31",
+    sha1 = "1a926b0cb2879fd32efbb3716ee8bab040f4218b",
 )
 
 maven_jar(