Can't wait to use the new JSONB operators of 9.5, e.g.:

    SELECT '{"k1":"v1"}'::JSONB || '{"k1":"v2","k2":true}'::JSONB

=> {"k1": "v2", "k2": true}

In 9.4 this is the 'best way' I know:

    SELECT
        ('{' || STRING_AGG(
	    '"' || COALESCE(j2.key, j1.key) ||
	    '": ' || TO_JSON(COALESCE(j2.value, j1.value)
        ), ',') || '}')::JSONB
    FROM JSONB_EACH('{"k1":"v1"}') j1
    FULL OUTER JOIN
    (SELECT * FROM JSONB_EACH('{"k1":"v2","k2":true}')) j2 ON j1.key = j2.key