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.
Created gripper firmware build date function.

This change avoids breaking firmware_version() behind the scenes.
Added an extra version check for legacy grippers.
Changed firmware thresh to be SDK release dates.
Merge pull request #45 from RethinkRobotics/gripper_date_check

Gripper check now uses firmware build date.