Show the name of the university with the lowest number of students in 2015.
lowest number of students refers to MIN(num_students); in 2015 refers to year = 2015; name of university refers to university_name;
SELECT T2.university_name FROM university_year AS T1 INNER JOIN university AS T2 ON T1.university_id = T2.id WHERE T1.year = 2015 ORDER BY T1.num_students ASC LIMIT 1