Moody's Investors Service has downgraded The Sage Colleges (NY) to a rating of a Caa1, making it the firm's lowest rated private college in the nation.  A combination of the college's dependence on an $8.5 million renewable line of credit from M&T Bank and a fall in its enrollment led to the low rating, a Moody's spokesman said. The rating, which previously was B3, has also come with a negative outlook.