currentCompatibleSbtVersion for sbt 0.13 is 0.13.0
Merge branch 'master' of github.com:jrudolph/sbt-cross-building
introduce `latestCompatibleVersionMapper` key to make mapping configurable

E.g. to update what the latest version of sbt should be from 0.13 to 0.13.0.
warn if using concrete versions instead of binary compatible one, refs #21

E.g. when using `^^0.13.0` instead of `^^0.13`
bump version to 0.8.1
update readme
next version
update self plugin to 0.8.1