Allow curl to follow redirects.

This is needed, for instance, to actually download a copy of Rubinius and not
GitHub's redirect page output which completely breaks RBX installations.
Stay tidy.
Merge pull request #36 from stefanozanella/follow_redirect

Allow curl to follow redirects.
Merge remote-tracking branch 'upstream/master' into skip_override_option
Don't be TOO smart.

Just refuse to install Ruby if another ruby executable is found in destination
directory.