Support scala.js 1.0.0-RC2

[Carlos Quiroz]

I tested it but maybe blocked by scala-js/scala-js#3924

[Miles Sabin]

I very much want to do this, but I'm currently a bit confused about sjs version compatibility ... I think I would need to add both 0.6.x and 1.0.0-RC2 to the build matrix?

@travisbrown you've been doing a lot of this recently ... any pointers?

[Lars Hupel]

Closed by #967