Wrong URL for SBT

When trying to download `sbt-launcher` we use obsolete URL which returns 404.

What worked for me is

```
wget https://github.com/sbt/sbt/releases/download/v0.13.18/sbt-0.13.18.zip
unzip sbt-0.13.18.zip -d ~
~/sbt/bin/sbt 'set test in assembly := {}' clean assembly
```

Is the repository still maintained? Should I submit a PR?