Travis-CI has an outdated version of sbt-extras, "repo.scala-sbt.org/scalasbt/maven-releases" no longer works

Our jobs require sbt 1.3.3 and Travis cannot download it because the redirect no longer works. We assume this applies to all other packages that are downloaded using the same link: https://repo.scala-sbt.org/scalasbt/maven-releases

Travis output:

Detected sbt version 1.3.3
Downloading sbt launcher for 1.3.3:
  From  https://repo.scala-sbt.org/scalasbt/maven-releases/org/scala-sbt/sbt-launch/1.3.3/sbt-launch-1.3.3.jar
    To  /home/travis/.sbt/launchers/1.3.3/sbt-launch.jar
Downloading sbt launcher 1.3.3 md5 hash:
  From  https://repo.scala-sbt.org/scalasbt/maven-releases/org/scala-sbt/sbt-launch/1.3.3/sbt-launch-1.3.3.jar.md5
  To  /home/travis/.sbt/launchers/1.3.3/sbt-launch.jar.md5
cat: /home/travis/.sbt/launchers/1.3.3/sbt-launch.jar.md5: No such file or directory
md5sum: 'standard input': no properly formatted MD5 checksum lines found
Checksum does not match
Could not download and verify the launcher. Obtain the jar manually and place it at /home/travis/.sbt/launchers/1.3.3/sbt-launch.jar

If you visit https://repo.scala-sbt.org/ you can see this message:

This is a redirection service for the sbt community repository. To insulate yourself from future migrations, please use the proxy URL instead of where it redirects (currently https://scala.jfrog.io/artifactory/)

Travis-CI has an outdated version of sbt-extras. Workaround: Use sbt -sbt-launch-repo https://repo1.maven.org/maven2

Imprint