switch to the official scrooge-sbt plugin

Author: @sprsquish
Fixes #312
URL: https://github.com/twitter/zipkin/pull/312
fix specs dependency

Author: @sprsquish
Fixes #317
URL: https://github.com/twitter/zipkin/pull/317
Merge remote-tracking branch 'origin/master' into zipkinweb-to-twitterserver