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.

cartodb.control.in

@@ -3,4 +3,4 @@ comment = 'Turn a database into a cartodb user database.'
 superuser = true
 relocatable = false
 schema = cartodb
-requires = 'schema_triggers, postgis'
+requires = 'schema_triggers, postgis, plpythonu'