Cross compile to scala 2.12

[OlegIlyenko]

Just verified - main codebase compiles with a few minor warnings (should be easy to fix).

Only 2 test dependencies are missing:

• moultingyaml - Cross-compile for Scala 2.12 jcazevedo/moultingyaml#16
• RxScala - Update Scala version to 2.12.0 final ReactiveX/RxScala#214

[OlegIlyenko]

Looks like some deps are still not on scala 2.12. Need to wait a bit.