Update url to leiningen (++)

Thanks for making this! I used it, but had to make a few changes to make it work. I thought you might like them too. :)

- Update the URL to leiningen. The old one redirects, but curl doesn't care about that.
- Avoid setting noninteractive in ENV, instead use parameter to dpkg-reconfigure
- Collapse RUN-statements to reduce number of file system layers generated when building.