Support for Kotlin 1.2 will be dropped with 1.1.0, Kotlin 1.3 with 1.2.0

[robstoll]

It's time to say goodbye to Kotlin 1.2. We drop support for Kotlin 1.2 with Atrium 1.1.0 and require Kotlin 1.3 as minimum version. And we will drop Kotlin 1.3 support with Atrium 1.2.0 and require Kotlin 1.4

Atrium still intends to support old Kotlin versions as long as possible as we know how troublesome it can be, especially for low budget projects, if one is forced to increase a language version due to libraries minimum requirements.