Upgrade nebula.netflixoss to replace bintray publication and update TravisCI Secrets

[rpalcolea]

Hi folks,

As you might be aware, JFrog is sunsetting Bintray and JCenter: https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

This upgrades to Nebula NetflixOSS which will publish to Nebula's OSS repos and Maven Central.

Repositories based on status:

• https://netflixoss.jfrog.io/artifactory/maven-oss-snapshots
• https://netflixoss.jfrog.io/artifactory/maven-oss-candidates
• https://netflixoss.jfrog.io/artifactory/maven-oss-releases

In addition:

• Fixed javadocs that caused build failures in modern versions of Gradle
• Fixed test dependencies
  • There was a dependency in test sources of a project which is invalid in Gradle now. I copied the artifacts for now as this is in maintenance mode, no need to modify the whole build to use modern gradle opinions