sprintf() uses %u instead of %d for unsigned integers.

https://github.com/pda/pheanstalk/issues/60 says:

> The beanstalkd protocol states that the priority is an unsigned int
> (up to 4294967295). Pheanstalk_Command_PutCommand->getCommandLine()
> (line 42) uses %d in the sprintf format string, which according to the
> PHP doc is '...presented as a (signed) decimal number'. As such, any
> priority over 2147483647 is formatted incorrectly. To correct this,
> the first %d in the sprintf just needs to change to %u.