Updating signing configuration for sbt 0.13.6.

The sonatype account information should be configured as sbt has documented it here:
http://www.scala-sbt.org/0.13/docs/Using-Sonatype.html

I've also used the command line gpg utility, and distributed a new key for signing. Something doesn't seem to work, though I suspect this is on the sonatype end, not the sbt side. We'll see.