Use Execution to run flows in REPL

- [x] Use Execution (from #915) to run REPL flows (rather than creating fake "Job"s)
- [x] Add unit tests for functionality added in #918 using Executions.
Why did you change the default version here? You can set it with: `./sbt ++2.10.4 compile` for instance. Until twitter moves to 2.10 internally, I think we should keep this at 2.9.