paycheck: support minor version 3.

Added check for mandatory source operation hash in minor version 3.

Fixed unittest.

BUG=chromium:568473
TEST=./paycheck.py <minor v3 payload>; ./checker_unittest.py

Change-Id: Id96ddce8c59f28b3449524b786c54a6c69ca798f
Reviewed-on: https://chromium-review.googlesource.com/317573


Trybot-Ready: Sen Jiang <senj@chromium.org>
Tested-by: Sen Jiang <senj@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>