don't depend on jcenter

[robstoll]

Non-Code related feature

Is your feature request related to a problem? Please describe.
Bintray/jCenter shuts down https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

Describe the solution you'd like
download everything from maven central, ping libraries we use which are only available from jcenter. So far detected:

• spek: Move artifact publishing directly to Maven Central spekframework/spek#949

[robstoll]

Looks like spek was the only dependency which as jcenter only and this one is synced by now. Was able to merge #804