Pass --fail to curl when downloading kerl

This causes curl to exit with a non-zero exit code on failure.
Update URL for kerl download

It has moved from raw.github.com to raw.githubusercontent.com.  There is
a 301 permanent redirect, but because we're not passing the --location
flag to curl, we didn't pick it up.
Merge pull request #12 from legoscia/kerl-url

Update URL for downloading kerl