Use values from per-subproject config when starting ensime-server

When starting the ensime-server for a sub-project we should use that project's configuration values. This includes :scala-version, :server-env, :name, :buffer, :java-home and :java-flags.

Older versions of ensime-sbt stored a short version string in :scala-version (e.g 2.10 rather than 2.10.4). These short versions are not compatible with the new sbt based launcher. Short version strings are now turned into valid Scala version strings.

Fixes #506.