Fix potential infinite loop in CDB_QueryStatements

This fix was already present at one point in cartodb/lib/sql
(where the code was copied from) but in a different branch than
the one the code was initially copied from.

The fix depends on plpython language which becomes a new dependency.

README.md

@@ -11,7 +11,8 @@ See https://github.com/CartoDB/cartodb/wiki/CartoDB-PostgreSQL-extension
 Dependencies
 ------------
 
- * PostgreSQL 9.3+ 
+ * PostgreSQL 9.3+ (with plpythonu extension)
+ * [PostGIS extension](http://postgis.net) 
  * [Schema triggers extension]
    (https://bitbucket.org/malloclabs/pg_schema_triggers)
    (or [fork](https://github.com/CartoDB/pg_schema_triggers))