erp5: Add jsonschema egg.

Pin to currently-published version, although there is a known (minor)
floating-point precision bug with multipleOf, fixed upstream but not
released yet.

stack/erp5/buildout.cfg

@@ -493,6 +493,7 @@ eggs =
   pprofile
   pycountry
   xfw
+  jsonschema
 # Needed for checking ZODB Components source code
   pylint
 
@@ -670,6 +671,7 @@ huBarcode = 1.0.0
 interval = 1.0.0
 ipdb = 0.8
 ipython = 2.3.1
+jsonschema = 2.4.0
 logilab-common = 0.63.2
 minitage.paste = 1.4.6
 minitage.recipe.egg = 1.107