stack/erp5: version up Zope 2.13.28.

stack/erp5/zope-versions.cfg

 [versions]
+Zope2 = 2.13.28
 AccessControl = 2.13.16
 Acquisition = 2.13.12
 DateTime = 2.12.8
@@ -29,7 +30,6 @@ Sphinx = 1.0.8
 ZConfig = 2.9.3
 ZODB3 = 3.10.7
 ZServer = 3.0
-Zope2 = 2.13.27
 ZopeUndo = 2.12.0
 docutils = 0.12
 initgroups = 2.13.0
@@ -41,6 +41,7 @@ pytz = 2017.2
 repoze.retry = 1.2
 repoze.tm2 = 1.0
 repoze.who = 2.0
+six = 1.11.0
 tempstorage = 2.12.2
 tox = 2.9.1
 transaction = 1.1.1