Placing a uniqueness constraint on a date range

I have a table reservation with columns roomno(INTEGER), startdate(DATE), enddate(DATE) with a primary key on (roomno, startdate). How do I place a constraint on the table such that bookings are not allowed to overlap? I am trying to implement this in SQLFIDDLE postgreSQL9.3 For example: 101 2016-01-01 2016-01-05 101 2016-01-03 2016-01-06 [This row should not be possible to insert] startdate and enddate are data type date.