Use ensime root-dir to find the sbt project path

I have a couple more pull requests...

If the sbt root-dir auto-detection doesn't work (for instance if there's not .sbt file in the root dir), we get dumped into "/". To prevent that I added a comparison with ensime's idea of the root dir.

Questions:
1) would it make sense to always use ensime-configured-project-root instead of guessing?
2) If not, would it be better for ensime-sbt to prompt for a directory (with the result of ensime-sbt-find-path-to-project as default)?