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.
Sorry, we can't do this, we need a release before we'll use the Git repo. Also, is there a source indicating that this is the "official" upstream repo?