Download older launchers if the version is declared in `build.properties`
Set `sbt.global.base` expectedly
add tests for sbt-extras based on sstephenson/bats

[Bats](https://github.com/sstepehnson/bats) is a testing tool for bash
scripts. It has been used in famous bash projects like Ruby's rbenv.
unset system's `JAVA_HOME` to force tests to use stubs
paranoiac string quoting
add .travis.yml