I think avrdude uses -b 115200 by default.
Currently there is a signed driver for Yosemite on this website:
https://www.mac-usb-serial.com/
which costs several dollars. However, CodeBender.cc and I have collaborated to develop a signed driver for all versions of Mac. I am not allowed to release it to the public yet, but if you email to [email protected] I can send you the driver directly.