@Chris-McCaslin Try using any pins other than 20 and 21 since they run the I2C protocol.
With regards to the servo, while fast-gpio is pretty handy, it's still software-based PWM. Anytime another process gets the CPU's full attention, the PWM signal will change. This will happen many times a second, so the PWM signal going to the servo will be constantly changing. This change in the PWM signal is the reason why your servo is twitching. Sorry, but your best bet for a non-twitchy servo is the Servo Expansion.
Another thing, your servo accepts a high pulse of 1ms to 2ms in a 20ms PWM period.
For the farthest position in one direction you would send a signal that is 1ms high and 19ms low, meaning a 5% duty cycle
The farthest position in the other direction requires a signal that is 2ms high and 18ms low, so a 10% duty cycle
And the neutral position would be: 1.5ms high and 18.5ms low, so 7.5% duty cycle
Andrei - I don't know if this helps, but your comments were so helpful I wanted to mention this. I've had ruby installed and didn't see a lot of additional dependencies. I just now installed ruby-irb and saw that opkg installed a whole bunch of ruby-related stuff.