Use errno.EDEADLK instead of errno.EDEADLOCK

EDEADLOCK doesn't exist on Darwin, but EDEADLK is equivalent on
Linux.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #5 from bfirsh/fix-edeadlock-not-on-os-x

Use errno.EDEADLK instead of errno.EDEADLOCK