Fix AttributeError 'PLyPlan' object has no attribute 'execute'

[drewdogg]

Trying to run the aws_s3.table_import_from_s3 function on PostgreSQL 9.x causes the following error:

ERROR:  AttributeError: 'PLyPlan' object has no attribute 'execute' (plpy_elog.c:121)
CONTEXT:  Traceback (most recent call last):
  PL/Python function "table_import_from_s3", line 7, in <module>
    return plan.execute(
PL/Python function "table_import_from_s3"

According to the current documentation, plpy.execute(plan [, arguments [, max-rows]]) can be called instead, which is compatible with current and older versions of Postgres.