Updates to the community build files

- modified the change of vars.base.extra.commands, so that the
original definitions of vars.base and vars.base.extra in
the community*.dbuild files are preserved

- added scoverage, now needed by specs2

- added scala-logging-slf4j and scala-logging-api (v2.1.2),
needed by scoverage

- updated 0.13.5-RC2 to 0.13.5 final (RC2 has been withdrawn)

- disabled root subproject in sbt, to work around the change
in publishing mechanism (now using Release)