Gripper check now uses firmware build date.

Gripper version 1.1.242 is the legacy gripper version,
and other 1.x.x grippers are invalid firmware versions. To avoid the
clashing of Manufacturing 1.x.x grippers and RSDK 1.1.x gripper
versions, I switched the check use gripper firmware dates. That way, any
recently built manufacturing or RSDK gripper will pass without warning
or error. Older build versions will warn, and any date as far back as a
bad firmware version will fail.

Also, removed deprecated Gripper.hardware_version() function.
The date is not the version; it is only what we are using to check compatibility now.  Don't change the functionality of this API behind the scenes (not to mention break with the meaning of the base ROS API) - make a new function if desired to access the date, or just make a deepcopy of the prop value directly.