sbt: cross project
Since recent updates of sbt, it started to give this warning:

```
/home/travis/build/kaitai-io/kaitai_struct/compiler/build.sbt:22: warning: method crossProjectFromBuilder in trait CrossProjectExtra is deprecated (since 0.6.23): The built-in cross-project feature of sbt-scalajs is deprecated. Use the separate sbt plugin sbt-crossproject instead: https://github.com/portable-scala/sbt-crossproject
lazy val compiler = crossProject.in(file(".")).
                    ^
```

It looks like we need to migrate to https://github.com/portable-scala/sbt-crossproject