Completion expiry should use timestamp instead of stored

[garemoko]

Comparing the expiry to a timestamp is better than to the stored date because what really matters is when the learner did the thing, not when it was stored in the LRS.

Modify tincanlaunch_get_completion_state in lib.php to not just check if statements are returned but also cycle through the statements and make sure that at least one has a timestamp greater than the expiry date.

Currently the relevant code looks like this:

        // If the statement exists, return true else return false.
        if (!empty($statementquery->content) && $statementquery->success) {
            $result = true;
        } else {
            $result = false;
        }

Note: setting the result variable to true/false and then returning the variable is unneeded. We can just return true or false.

[davidpesce]

Fixed in #198