I don't know why it mentions Adafruit, there is no mention of it in the code.
/Users/brianmatthews/Documents/Arduino/libraries/DHT_sensor_library-1.3.0/DHT_U.h:25:29: fatal error: Adafruit_Sensor.h: No such file or directory
#include <Adafruit_Sensor.h>"
But it does get a mention, indeed it is #included by DHT_U.h