Cannot inline bytecode built with JVM target 1.8 into bytecode that is being built with JVM target 1.6
## About this issue

`4.0.0-rc02` is being shipped with Kotlin code that requires Java 1.8. This causes the following compiler error:

```
Cannot inline bytecode built with JVM target 1.8 into bytecode that is being built with JVM target 1.6
```

I believe upgrading to Kotlin 1.3.31 should fix the issue.

## Details
- [ ] Used library version
- [ ] Used support library version
- [ ] Used gradle build tools version
- [ ] Used tooling / Android Studio version
- [ ] Other used libraries, potential conflicting libraries

## Checklist

- [x] Searched for [similar issues](https://github.com/mikepenz/Android-Iconics/issues)
- [ ] Checked out the [sample application](https://github.com/mikepenz/Android-Iconics/tree/develop/app)
- [x] Read the [README](https://github.com/mikepenz/Android-Iconics/blob/develop/README.md)
- [x] Checked out the [CHANGELOG](https://github.com/mikepenz/Android-Iconics/releases)
- [x] Read the [FAQ](https://github.com/mikepenz/Android-Iconics/blob/develop/FAQ.md)
- [x] Checked out the [MIGRATION GUIDE](https://github.com/mikepenz/Android-Iconics/blob/develop/MIGRATION.md)
