Recurrent Scala build environment regression around sbt-extras (Download failed. Obtain the jar manually)

In https://travis-ci.org/typelevel/jawn/builds/464581687 I just saw

0.25s$ sbt "+ test"
Downloading sbt launcher for 1.2.7:
  From  http://repo.typesafe.com/typesafe/ivy-releases/org.scala-sbt/sbt-launch/1.2.7/sbt-launch.jar
    To  /home/travis/.sbt/launchers/1.2.7/sbt-launch.jar
Download failed. Obtain the jar manually and place it at /home/travis/.sbt/launchers/1.2.7/sbt-launch.jar
The command "sbt "+ test"" exited with 1.

This is a really old bug of sbt-extras that was fixed in May of 2016 (https://github.com/paulp/sbt-extras/pull/150), since sbt 1 launchers are no longer published to repo.typesafe.com.

For some reason, sbt has been working ok so maybe there’s some other process that’s happening to grab a later version of sbt-extras, but it seems to periodically revert back to this one.

Side note

See https://github.com/travis-ci/travis-ci/issues/9816.

There are two pending pull requests:

I think this was actually caused by my removing language: scala from .travis.yml by mistake:

The odd thing is that language: ruby that the build fell back to still contained sbt but just a stale version of one.

The confusion comes from the fact that Travis CI includes old version of sbt-extras in travis-cookbooks - https://github.com/travis-ci/travis-cookbooks/blob/509637e5028550df15b3691292b9d6dfe32edaef/cookbooks/travis_sbt_extras/attributes/default.rb

Could we just remove it from cookbooks, so if you don’t specify language: scala it will just not install any sbt, extras or otherwise?