Add sbt to ci

[NavidJalali]

The runners no longer have sbt by default

[sjrd]

Thanks for the PR. In order to maintain consistency between CI setups in the scala-js organization, I submitted and merged #29 instead. Also, that PR does not unnecessarily bump JDK and Node.js versions. Bumping versions in CI scripts like that essentially means dropping support for the older versions (since they are now untested), which we do not do lightly.