improve build process, part of #481

fix oneJar code coverage issue (coverage info corrupts JAR), test built SlamEngine JAR, allow environment variable overrides of backend-config, modify travis to use local mongodb override, auto-import zips data set when in travis builds. note: publish script has not been updated and jenkins is no longer expected to work.
Looks like we're missing a step here to start mongod and/or create the `test` db:
https://travis-ci.org/slamdata/slamengine/builds/44116639#L352