Update sbt-extras script

Former version of sbt-extras launcher has a bug in comment skipping of
jvmopts/sbtopts files.

We also only pre-install Scala final releases, but no milestone or
release candidate. We'll also have to evaluate to remove some old
versions.

Note: At the moment, we can safely keep the sbt cookbook at version
0.3.0 (upgrading to 0.4.0 can wait)