Upgrade to SBT 0.13 and enable code coverage

Title says it all.    You can try it by running `scoverage:test` command in SBT.

After the newer version of socrata-cloudbees-sbt plugin is released, then the version in `project/build.sbt` needs to be updated.
Version 1.2.4 that is currently built and released is due to an unrelated bugfix I made. Evan's changes are still not merged, and will require another version bump above 1.2.4