Upgrade Truth to 0.28

This includes the new MultimapSubject#containsExactlyEntriesIn method
which we were previously consuming from the snapshot version.

Change-Id: Ieb3bc9c31e58c6ca687bd6938b7ca30cb457ead3

lib/BUCK

diff --git a/lib/BUCK b/lib/BUCK
index abde92f..a14b356 100644
--- a/lib/BUCK
+++ b/lib/BUCK
@@ -87,10 +87,8 @@
 
 maven_jar(
   name = 'truth',
-  id = 'com.google.truth:truth:1.0-20151120.141336-28',
-  sha1 = 'c40c13431eedde9ed168ee908af8891bf1a09245',
-  repository = GERRIT,
-  attach_source = False, # Remove when using non-snapshot version.
+  id = 'com.google.truth:truth:0.28',
+  sha1 = '0a388c7877c845ff4b8e19689dda5ac9d34622c4',
   license = 'DO_NOT_DISTRIBUTE',
   exported_deps = [
     ':guava',