Upgrade to sbt 0.13 / socrata-cloudbees-sbt 1.3.0 / rojoma-json 2.4.3.

Enables code coverage support via sbt scoverage:test.
Merge pull request #2 from socrata/marcs/codecoverage

Upgrade to sbt 0.13 / socrata-cloudbees-sbt 1.3.0 / rojoma-json 2.4.3.