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.
It's probably okay, but may be good to double-check for legacy with the version number.