Change implementation of StatementElement.

Instead of having both next and execution as attributes, we now only have next, because it's an execution if there's an array.

jedi/cache.py

@@ -243,7 +243,7 @@ def save_parser(path, name, parser, pickling=True):
 
 class ParserPickling(object):
 
-    version = 14
+    version = 15
     """
     Version number (integer) for file system cache.