Circleci (#920)

Be consistent with the use of /dev/null for sbt's stdin (force sbt to exit instead of bringing up a dialog).
Enforce Jenkins' scalastyle limit of 40 warnings via CHECKSTYLE_LIMIT in environment.
Force tests to run sequentially if -DminimalResources is set on the command line.
Ensure we see valid scalastyle output.
Make checkstyle dependent on one of the tests (so a style failure will fail the build only after tests pass).