Support for postgreSQL 10 and 11

[lsoica]

For Postgresql 10 and 11 and plv8 2.3.9 (latest at the moment of writing), running plv8 -i after successfully running a plv8 --list returns:

2018-12-16 15:53:04.155 UTC [98] ERROR:  function "array_to_json" already exists with same argument types
2018-12-16 15:53:04.155 UTC [98] STATEMENT:
        SET client_min_messages TO WARNING;
        DO $PLV8X_EOF$ BEGIN

        DROP FUNCTION IF EXISTS array_to_json (__0 anyarray) CASCADE;
        EXCEPTION WHEN OTHERS THEN END; $PLV8X_EOF$;

        CREATE FUNCTION array_to_json (__0 anyarray) RETURNS plv8x.json AS $PLV8X__BODY__$
        ;
        return JSON.stringify((eval((function () {return (function(it){
          return it;
        }) })()))(__0));
        $PLV8X__BODY__$ LANGUAGE plv8 IMMUTABLE STRICT;
/plv8x/lib/index.js:29
        throw err;
        ^

Error: ERROR:  function "array_to_json" already exists with same argument types

    at module.exports.Client._readError (/plv8x/node_modules/pg-native/index.js:80:13)
    at module.exports.Client._read (/plv8x/node_modules/pg-native/index.js:121:19)
    at emitNone (events.js:106:13)
    at PQ.emit (events.js:208:7)