Pin ipython to avoid incompat >=6 release

Since release of ipython 6.0.0 (
https://pypi.python.org/pypi/ipython/6.0.0 ) ipython became incompatible
with python < 3.3, so its version needs to be pinned.

software/erp5testnode/software.cfg

@@ -62,3 +62,4 @@ md5sum = 05519f3887a309d3ec069e0aa9f52ebc
 PyXML = 0.8.5
 erp5.util = 0.4.47
 slapos.recipe.template = 2.7
+ipython = 5.3.0