revert version down of zope.interface for Python 2.6 compatibility.

because zope.interface is not required to build rebootstrap python.

stack/slapos.cfg

@@ -190,8 +190,7 @@ uritemplate = 3.0.0
 
 # Required by:
 # slapos.core==1.3.18
-# zope.intercace >= 4.2.0 drops support for Python 2.6
-zope.interface = 4.1.3
+zope.interface = 4.3.2
 
 [networkcache]
 download-cache-url = http://www.shacache.org/shacache