switch to sbt-git

[SethTisue]

at #35 @lrytz suggests: "Could we switch to https://github.com/sbt/sbt-git? Then we don't need to keep updating the version after each release. Here's how it is configured in scala-asm: https://github.com/scala/scala-asm/blob/master/build.sbt#L5-L7"

[SethTisue]

these days there's also @dwijnand's https://github.com/dwijnand/sbt-dynver

[SethTisue]

obsoleted by scala/scala#6566