It doesn't mention which version of PostgreSql. I hope it is 9.3. It's got to be at least 9.2 since it supports the Json datatype.
In PG 9.2 there were only two json functions[1], but 9.3 introduced more[2]
[1]http://www.postgresql.org/docs/9.2/static/functions-json.htm... [2]http://www.postgresql.org/docs/9.3/static/functions-json.htm...