Add Gripper-attach Prompt to Update Tool

Adds prompt to attach gripper to update tool before running update.
Gripper Firmwares are upgraded during the robot update process if
they are attached -- this is recommended operating procedure.
Otherwise, users must go into the robot FSM to Update Gripper
Firmware.

NOTE: raw_input() is not Python 3 compatible; nor does it exit
      immediately upon KeyboardInterrupt (Ctrl^C)
Merge pull request #14 from RethinkRobotics/update_prompt

Add Gripper-attach Prompt to Update Tool