openjdk8 + Scala 2.13.1 + javafx/scalafx

[metaphori]

See this build failure on Travis CI

https://travis-ci.org/github/scafi/scafi/jobs/673025035

The problem is that for Scala 2.13 I needed to raise scalafx version to 12 which however supports Java 11+ (I handled conditional compilation against scalafx 8 for java 8 and scalafx 12 for java11+).

As dealing with this issue would require to exclude scalafx subprojects from TravisCI job (openjdk8,scala2.13), hence requiring ad-hoc, conditional build code, I am temporarily dropping java 8 support.

[DanySK]

This is solved, I think. @cric96 appears to have filtered out the required JFX version selectively