Fix download url.

Change download url from `http://code/tools...` to `http://code.icecube.wisc.edu/tools...` in order to make it work without having `code` defined in the `/etc/hosts`.

In order to try this out, just:

```bash
brew install https://raw.githubusercontent.com/fiedl/homebrew-icecube/b56ddc66780d50c0d0b8ec2af7d23d2add36bd8f/slalib-c.rb
```