Remove scalaVersion from built.sbt

[stevedlawrence]

From the SBT Plugin Docs

• sbt plugins must be compiled with Scala 2.12.x that sbt itself is compiled in. By NOT specifying scalaVersion, sbt will default to the Scala version suited for a plugin.
• By default sbt plugin is compiled with whichever the sbt version you are using. Because sbt does NOT keep forward compatibility, that would typically require all of your plugin users to upgrade to the latest too. pluginCrossBuild / sbtVersion is an optional setting to compile your plugin against an older version of sbt, which allows the plugin users to choose from a range of sbt versions.

So we should probably shouldn't set scalaVersion at all. I think setting crossSbtVersion like we do now is sufficient.

Originally posted by @stevedlawrence in #53 (comment)