Update Kotlin to 1.3.0

[AlvaroBrey]

Kotlin 1.3 is now stable, so update to that version.

This, however:

• Needs careful review to verify everything works as expected
• Needs an upgrade of the AS plugin that isn't offered in the usual way (have to go through Settings).

Release notes: https://blog.jetbrains.com/kotlin/2018/10/kotlin-1-3/