Add Docker and Travis config files to enable CI tests

Also fixes some Rails 6.0 deprecation warnings

app/logical/moderator/dashboard/queries/artist.rb

@@ -7,7 +7,7 @@ module Moderator
             .where("artist_versions.created_at > ?", min_date)
             .where("users.level <= ?", max_level)
             .group(:updater)
-            .order("count(*) desc")
+            .order(Arel.sql("count(*) desc"))
             .limit(10)
             .count
             .map { |user, count| new(user, count) }