`travis/default.yml` doesn't work with sbt 1.4.8

[SethTisue]

the travis/default.yml in this repo, which lots of repos inside and outside the Scala org use, doesn't work with sbt 1.4.8

thus many of the Scala Steward's sbt 1.4.8 upgrade PRs are failing

[SethTisue]

I assume the problem here is that Travis-CI provides an outdated version of sbt-extras

one thing we could do is report that upstream and hope they fix it, but actually I think we ought not to be using sbt-extras at all, we ought to be using the official sbt launch script

[eed3si9n]

I assume the problem here is that Travis-CI provides an outdated version of sbt-extras

Detected sbt version 1.4.8
Downloading sbt launcher for 1.4.8:
  From  https://repo.scala-sbt.org/scalasbt/maven-releases/org/scala-sbt/sbt-launch/1.4.8/sbt-launch-1.4.8.jar
    To  /home/travis/.sbt/launchers/1.4.8/sbt-launch.jar
Downloading sbt launcher 1.4.8 md5 hash:
  From  https://repo.scala-sbt.org/scalasbt/maven-releases/org/scala-sbt/sbt-launch/1.4.8/sbt-launch-1.4.8.jar.md5
    To  /home/travis/.sbt/launchers/1.4.8/sbt-launch.jar.md5
cat: /home/travis/.sbt/launchers/1.4.8/sbt-launch.jar.md5: No such file or directory

@xuwei-k fixed this already! dwijnand/sbt-extras#305

ok so you're right. It's a problem with sbt-extras, which is now fixed.

[SethTisue]

perhaps I ought to first do a more minimal fix where I grab the latest sbt-extras and overwrite the Travis-CI one

and then separately consider whether to get off sbt-extras. if sbt-extras is the Travis default, then it might be surprising and/or breaking for some downstream users for us to have a different default here

[SethTisue]

I verified at lightbend-labs/scala-sculpt#142 that the fix works