Fix steering bug (#394)

Author: gregcowell

ev3dev/motor.py

@@ -1538,9 +1538,9 @@ def on_for_rotations(self, left_speed_pct, right_speed_pct, rotations, brake=Tru
 
         if left_speed > right_speed:
             left_rotations = rotations
-            right_rotations = float(right_speed / left_speed) * rotations
+            right_rotations = abs(float(right_speed / left_speed)) * rotations
         else:
-            left_rotations = float(left_speed / right_speed) * rotations
+            left_rotations = abs(float(left_speed / right_speed)) * rotations
             right_rotations = rotations
 
         # Set all parameters