Migration notes

Build definition is based on Scala 2.12.1
Build.scala style builds are gone. Use multi-project build.sbt.
Project(...) constructor is restricted down to two parameters. Use project instead.
sbt.Plugin is also gone. Use auto plugins.
The incremental compiler, called Zinc, uses class-based name hashing.
Zinc drops support for Scala 2.8.x and 2.9.x.
Removed the pre-0.13.7 *.sbt file parser (previously available under -Dsbt.parser.simple=true)
Removed old, hyphen-separated key names (use publishLocal instead of publish-local)
Removes no-longer-documented old operators <<=, <+=, and <++=.
Renames early command feature from --<command> to early(<command>).
Log options -error, -warn, -info, -debug are added as shorthand for "early(error)" etc.
sbt.Process and sbt.ProcessExtra are gone. Use scala.sys.process instead.