Timezone switch bug

[MirkoBonadei]

The function \Recruiter\RetryPolicy\TimeTable::estimateHowManyRetriesIn assumes that the default timezone is UTC (which should be the best practice).

In case the default timezone is different than UTC (please refer to date_default_timezone_get() to known the default time zone of your PHP runtime) the estimateHowManyRetriesIn will fall in a subtle bug which causes a time travel in the future when an amount of time is subtracted from another instance of time (at [1] you can read one of the many reports about this bug).

Please note that supporting only UTC is a good idea. Maybe we should just check the result of the function date_default_timezone_get and if it is different from UTC we should throw an exception.

[1] - https://bugs.php.net/bug.php?id=68286