I've always been a huge fan of Code Climate, and have been using it for several years. The platform offers automated code review for test coverage, complexity, duplication, security, style, and more...
I don't have any personal experience with Hound CI, but this is built by the great folks at Thoughtbot which are known for building great projects, so it's def worth checking out!
PullRequest is an on-demand service for code review from experts. It seems to fit your specs exactly.
disclaimer: I'm on the team building PullRequest :)