tox: Limit ZODB4 to be <5.0

Now that ZODB 5.0 eggs are starting to appear (see e.g. [1] for context)
let's limit ZODB4 test setup to actually install ZODB4, not 5.

[1] https://groups.google.com/forum/#!topic/zodb/P05S0pyUbAM

tox.ini

@@ -13,7 +13,8 @@ deps    =
     ZODB3:  ZODB3 >=3.10, <3.11dev
 
     # latest current ZODB _4_
-    ZODB4:  ZODB3 >=3.11
+    ZODB4:  ZODB >=4.0, <5.0dev
+    ZODB4:  ZEO  >=4.0, <5.0dev
 
     neo:    neoppod >= 1.6.2
     # neo.tests also imports MySQLdb unconditionally