lockrun: Changed to github repo.

Fixes a bug: now correctly passes on the exit status. The previous version would always return a 0 exit status even if the command failed.
Reproduce bug:

lockrun --lockfile=/tmp/lock -- false
echo $?

Should return 1.
Ah ok. It's probably not the official one, but the official one contains a bug. I guess I should have it fixed in the official one then?