Crossbuild for sbt 2.0

[limansky]

sbt 2.0.0-M2 is released. sbt-git-flow-version should support sbt 2.0, so it should be crossbuilded.

It have to be done after sbt-git will be released (sbt/sbt-git#253).

[BillyAutrey]

sbt-git is available in https://github.com/sbt/sbt-git/releases/tag/v2.1.0 with 2.0.0 crossbuilds.

[limansky]

@BillyAutrey Thanks. I'm almost done with cross build.