Support for RSpec Junit reporting

[phoffer]

This adds support for Junit reporting. It seems the current design of turbo tests is not able to work with RSpec Junit, no matter what args or configuration I try. This is important for CircleCI. I work on a couple projects using parallel_tests in Circle, but it would be nice to have consolidated log output with failures listed together, as turbo_tests does. However, switching to turbo_tests removes the failing tests from the Tests tab in Circle (see working example below). Getting RSpec Junit to work makes it possible to use turbo tests in Circle and still extract the failures into that tab