bug fix for running spring tests under rails >= 4.0 without spring-comma...

Just installed Rails 4.1 and enabled the built-in Spring integration. Found that guard fails with and without passing the option `spring: true`. Without the the spring option, the default runner `ruby` doesn't recognize the `--use-color` switch. With the spring option, it didn't recognize the `testunit` command. It turns out you don't need spring-commands-testunit under rails 4.x since `rake test` can accept a path as an argument.

This patch just does some rails version sniffing and changes the command appropriately.