Update Readme with latest ensime-sbt version
Update Readme with latest ensime-sbt version

Added a note to use sbt 0.13.9 and remove unnecessary sonata repository
from the instructions.

Finally removed a link to a non existent page and added a link to
ensime-sbt GitHub page for further instructions.

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs