Drop scalaVersionsByJvm and add sbt-travisci

[Philippus]

Applying this PR will:

• drop scalaVersionsByJvm because we have the alternative of tag based publishing
• add sbt-travisci to base the values for crossScalaVersions and scalaVersion on the scala versions defined in the travis.yml file

projects still using scalaVersionsByJvm listed below, maybe migrate them to tag based publishing first:

• scala-async
• scala-collection-compat
• scala-continuations
• scala-parallel-collections
• scala-partest
• scala-swing

ping @SethTisue @dwijnand

[lrytz]

👍 I wanted to do that but didn't push it through all the repos...

add sbt-travisci

Does this also set the scala version outside travis, based on .travis.yml?

scala-xml seems to be (also?) using circle-ci, but i think it would be good to standardize on travis for all modules.

[dwijnand]

Does this also set the scala version outside travis, based on .travis.yml?

Yes. The things that are specific to running on Travis CI are:

• isTravisBuild in Global will be automatically be set to true if the current build is running under Travis CI.
• travisPrNumber in Global will be automatically set to the number of the pull request, in case the build is a pull request build.
• scalaVersion in ThisBuild will be automatically set to TRAVIS_SCALA_VERSION if isTravisBuild is true, otherwise to the last version in crossScalaVersions, and so by default, sbt will assume you want to develop under the last version listed in .travis.yml.

(That last one turned out to be the wrong fallback behaviour alas...)

[lrytz]

I continued this in #65. Thanks, @Philippus